French Onion Grilled Cheese
Make a tasty French Onion Grilled Cheese sandwich with caramelized onions, melted Gruyère, and peppery fresh arugula all layered between buttery slices of toasted French bread! It's all the deep, savory comfort of the classic soup in sandwich form. Best of all, the drizzle of balsamic glaze at the end ties every layer together. Enjoy it on its own or paired with your favorite bowl of soup.
Quick Glance at French Onion Grilled Cheese Recipe
French Onion Grilled Cheese At a Glance
Everything you need to know before you start
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I have a confession to make: the first version of this recipe was a disaster. I made a homemade onion jam for the filling - deeply caramelized, intensely reduced, beautiful in theory - and it completely overwhelmed everything else in the sandwich. So I went back to the drawing board, and three rounds of testing later, I landed on something I actually want to eat on repeat.
The onions are cooked low and slow with just a touch of sugar, balsamic vinegar, thyme, and a bay leaf. They're savory-sweet and jammy without being candy. That balance is the whole recipe. And, a small handful of peppery greens in the sandwich cuts through the richness of the cheese and butter in a way that makes the whole thing feel lighter without losing any of the comfort.
If you love French onion soup, this sandwich will feel immediately familiar. For more unique grilled cheese recipe ideas, try my pizza grilled cheese and brie grilled cheese too!
Why You'll Love This Recipe
- Caramelized onions, done right. Made over low heat with balsamic vinegar, thyme, and a bay leaf for a flavor that's savory, complex, and nothing like the shortcut versions.
- Arugula inside the sandwich. Peppery greens cut the richness of the cheese and butter in the best possible way. A unique touch you won't find in other recipes!
- Tested three times to get the balance right. The sugar level matters more than you'd think - one teaspoon is the sweet spot.
- Onions can be made days ahead. Make a big batch on the weekend for a very fast weeknight dinner.
Ingredients + Substitutions
Here's what goes into this sandwich and why each one matters:
- Onion: Yellow onion is the easy choice while sweet onions like Vidalia will caramelize a little faster and taste slightly milder.
- Butter and olive oil: The combination gives you richness from the butter and a higher smoke point from the oil, allowing the onions to cook low and slow without burning the butter. You will also need butter on the bread before grilling your sandwich in the pan.
- Sugar. One teaspoon is enough to assist caramelization without overpowering the savory notes.
- Balsamic vinegar. Added to the onions partway through cooking, it adds depth, acidity, and a slight sweetness that tastes nothing like plain sweet onions.
- Dried thyme and a bay leaf. The herb combination is what pushes the onions firmly into French onion soup territory. Don't forget to remove the bay leaf before assembling.
- Bread. Country or ciabatta-style bread both work. These are both sturdy enough to hold up to the onions, with a neutral flavor that allows the cheese and caramelized onions to shine through. If your slices are very thick (over an inch), drop the heat to medium-low so the bread doesn't brown before the cheese melts.
- Gruyère cheese: Gruyère is earthy, nutty, and melts evenly without becoming greasy or stringy. Buy a block and grate it yourself for the best flavor and smoothest melt.
- Arugula. Baby arugula is milder and slightly more delicate. Standard arugula has more of a peppery punch.
Check the recipe card below for the full ingredient list and quantities of each one.
French Onion Sandwich Variations
- Cheese Swaps: Emmenthal and Comté are the most natural swaps and the closest in flavor. Fontina is a good melter with a milder taste, and Swiss cheese works in a pinch.
- Change up the Bread: Try it with sourdough for a tang plays beautifully against the sweet onions, while brioche works if you want a richer and slightly sweeter flavor.
- Enjoy it Open-Faced: Arrange the cheese and onions on a single slice of thick French bread and broil until bubbly. This is very close to the crouton and cheese topping on a bowl of French onion soup, and just as satisfying. Try my prosciutto sandwich if you love open-faced sandwiches!
- Add a Protein: Thin slices of roast beef or leftover steak turn this into a considerably more substantial meal. Layer the meat over the caramelized onions before adding the second layer of cheese.
How to Make a French Onion Grilled Cheese
To get started making this French onion sandwich recipe, grab all your ingredients and your favorite heavy pan. A cast-iron or stainless steel skillet retains heat evenly and gives you good control for caramelizing the onions.
Step 1: Start Cooking Onions. Heat the olive oil and butter in a saucepan over medium-low heat. Add onions, sugar, and salt.
Step 2: Cook. Cook for 18-20 minutes until onions soften and begin to brown. At this point, the onions should be very soft and just beginning to turn golden at the edges. Be sure to stir occasionally so they brown evenly and don't burn. If they begin to brown too quickly, reduce the heat a bit.
Step 3: Add Seasonings. Add the balsamic vinegar, thyme, and bay leaf to the pan.
Step 4: Finish Cooking. Cook the onions for another 8-10 minutes. Onions should be very soft and golden brown when they're ready. Remove the bay leaf and add more seasoning to taste. Transfer to a paper towel-lined plate to drain any excess butter and oil to keep the sandwich from being soggy.
Step 5: Assemble Sandwich. Butter one side of all four slices of bread. Place two of them upside down in a cold pan or on a clean surface, and top with half the cheese.
Step 6: Add Onions. Add the onion mixture on top, spreading it out to the edges.
Step 7: Add the Arugula. Add the arugula and top with the remaining cheese. Finish it off with the other top slice of buttered bread with the butter side facing up.
Step 8: Cook the Sandwiches. Place the skillet over medium heat and cook until they brown on one side. Flip and continue to cook until the cheese is melted and both sides of the bread are golden brown. Serve as a side with a bowl of creamy red lentil soup.
Lyn's Top Tips
- Make the Onions Ahead: Caramelized onions keep beautifully in an airtight container in the refrigerator for up to 5 days. Making a double or triple batch at the start of the week turns this sandwich into a weeknight decision that takes under 15 minutes. They also freeze well for up to 3 months - freeze in small portions so you can thaw only what you need.
- Make Balsamic Glaze: Pour ½ cup of balsamic vinegar into a small saucepan. Bring to a gentle simmer over medium heat and cook, stirring occasionally, for 5-8 minutes until it reduces by about half and coats the back of a spoon. It will thicken more as it cools. Drizzle over the finished sandwich just before serving. A bottle of store-bought keeps indefinitely in the refrigerator.
- Start with a Cold Pan: Starting with a cold pan allows the bread to warm and the butter to melt gradually into it before browning begins, which means more even color and less risk of a burned exterior with an undermelted interior.
- Using Thick Bread Slices: This makes it more difficult for the cheese to melt before the bread browns. If you find that the bread is toasted but the cheese isn't melted, transfer the sandwich to a 400°F oven and toast for 5-8 minutes to melt the cheese.
- Storage: These sandwiches are best eaten immediately. If you have leftovers, wrap tightly and refrigerate for up to 2 days. To reheat, place on a sheet pan in a 375°F oven for 5-7 minutes until warmed through and the bread is crisp again. An air fryer at 350°F for 3-4 minutes works equally well. Avoid the microwave as it will make the bread soft and the filling watery.
Soup-er Serving Suggestions
- Serve alongside classic French onion soup for the full French onion experience. The sandwich is essentially a deconstructed version of the crouton-and-cheese topping.
- Pair with a simple tomato soup for the classic grilled cheese combination!
- This French onion grilled cheese pairs well with savory beef soups, like this easy beef barley soup and vegetable beef stew.
- Use extra caramelized onions to add to scrambled eggs or a frittata, or you can spread them on pizza dough in place of tomato sauce, topped with Gruyère or brie for an upscale pizza!
French Onion Grilled Cheese Recipe FAQs
The oven is the best option! To reheat, place leftovers on a sheet pan and cook in the oven at 375°F for 5-7 minutes until warmed through and crisp. An air fryer at 350°F for 3-4 minutes works equally well. Avoid the microwave - it will steam the bread soft and make the filling watery.
Any sturdy country-style loaf or ciabatta bread works well. It holds the filling without becoming soggy, and the sturdy crust crisps up beautifully. Sourdough is an excellent alternative, especially if you like a tangy contrast to the sweet onions. Avoid thin, soft sandwich bread because it won't hold the weight of the onion filling.
Yes, it's a great way to make this a fast weeknight dinner. Also, it's a great idea to make a large batch to use them in other dishes or for future meals. Store the caramelized onions in an airtight container in the refrigerator for up to 5 days. They also freeze well for up to 3 months. I like to freeze in small portions so I can grab just what I need. Remove the night before and thaw in the refrigerator overnight.
Plan on 26-30 minutes total. I find that it takes about 18-20 minutes for the onions to soften and begin browning, then another 8-10 minutes after adding the balsamic, thyme, and bay leaf. The finished onions should be very soft, deeply golden, and almost jammy. Do not rush the process using high heat. This causes the onions to brown unevenly and taste bitter instead of sweet.
French Onion Grilled Cheese
Ingredients
Caramelized Onions
- 1 tablespoon olive oil
- 1 tablespoon butter
- 1 large onion sliced thin (about 2-3 cups)
- 1 teaspoon sugar
- ¼ teaspoon salt plus more to taste if needed
- 1 tablespoon balsamic vinegar
- ¼ teaspoon dried thyme
- 1 small bay leaf or ½ large
Grilled Cheese
- 4 slices Country-style or ciabatta bread
- 2 tablespoons butter
- 1 cup grated Gruyere cheese Emmenthal or Comte are good substitutes
- 1 cup arugula
- Balsamic vinegar syrup optional
Instructions
Make Caramelized Onions
- Heat olive oil and butter in a saucepan over medium-low heat. Add onions, sugar, and salt. Cook for 18-20 minutes until onions soften and are beginning to brown.
- Add balsamic vinegar, thyme, and bay leaf. Cook for another 8-10 minutes. Onions should be very soft and golden brown. Remove bay leaf. Taste and add more seasoning if needed. Transfer to a paper towel-lined plate to drain any excess butter and oil.
Assemble Sandwich
- Butter one side of all slices of bread. Place upside down in cold pan or on a clean surface. Top half the slices with half the cheese. Add onion mixture on top, followed by arugula. Top with remaining cheese and then top with other slice of buttered bread with the butter side facing up.
- Place skillet over medium heat and cook until they brown on one side. Flip and continue to cook until cheese is melted and both sides of bread are golden brown.
