Heat a Dutch oven or large pot over medium heat with the olive oil and butter until the butter has melted. Add the onion, celery, and carrots, and saute for 6-7 minutes or until the onions are translucent.
Add the garlic and thyme. Sauté for 1 more minute, or until golden and fragrant.
Stir in the chicken broth and chicken bouillon (if using). Bring to a simmer and cook with the lid ajar for 10 minutes.
Stir in the orzo. Bring to a simmer and cook, uncovered, for 5 minutes, stirring occasionally to prevent sticking.
Stir in the chicken. Bring to a simmer and cook for 5 minutes or until the orzo is tender.
Reduce the heat to low. Stir in the lemon juice, lemon zest, parsley, and salt and pepper to taste. Add more lemon juice and zest to taste and serve hot!