Heat the butter in a large soup pot over medium heat until melted. Add the diced onion and red bell pepper and cook, stirring frequently, for 7-8 minutes or until the onions start to turn golden.
Add the garlic and cook, stirring consistently, for 30-60 seconds or until fragrant.
Remove the pan from the heat (the paprika can burn easily, making it bitter) and stir in the sweet paprika, salt, and pepper until well combined.
Stir in the chicken broth, canned tomatoes, and shredded chicken. Return the pan to high heat and bring the soup to a boil. Reduce the heat to medium and simmer uncovered for 10 minutes.
While the soup simmers, cook the noodles or spaetzle according to the package directions in heavily salted water. Drain well.
Whisk the sour cream, milk, and flour together in a small mixing bowl until smooth.
Add the sour cream mixture into the soup and stir very well to dissolve the sour cream. Bring to a simmer and cook for 3-5 minutes or until the soup has thickened. Use a whisk to break up any lumps in the soup that appear right after adding the flour mixture.
Stir in the cooked and drained noodles. Season the soup to taste with salt and cracked pepper, and serve hot, garnished with chopped flat-leaf parsley if desired. Enjoy!