**Preheat Oven**: Preheat your oven to 375°F (190°C).
**Caramelize Onions**: In a medium skillet, heat olive oil and butter over medium heat. Add the sliced onions and cook, stirring occasionally, for 20-25 minutes, until onions are golden and caramelized. Add balsamic vinegar and thyme, then cook for another 1-2 minutes.
**Prepare Sauce**: Sprinkle the flour over the onions, stirring to combine. Slowly add the beef broth while whisking to avoid lumps. Let the mixture simmer for a few minutes until it thickens slightly. Season with salt and pepper to taste.
**Assemble the Dish**: Place the chicken breasts in the baking dish and pour the onion sauce over the top. Sprinkle the shredded mozzarella and Gruyère cheeses evenly over the chicken and onions.
**Bake**: Place the dish in the preheated oven and bake for 25-30 minutes, or until the chicken is fully cooked (internal temperature should reach 165°F).
**Serve**: Garnish with fresh parsley if desired and serve the chicken bake hot, with a side of crusty bread or roasted vegetables.
### Prep Time:
- 15 minutes
### Cook Time:
- 50 minutes (including caramelizing onions and baking)
### Total Time:
- 1 hour and 5 minutes
### Nutrition (per serving):
- Calories: 400
- Protein: 36g
- Carbohydrates: 10g
- Fat: 25g
- Fiber: 2g
- Sodium: 700mg
- Sugar: 5g
This **French Onion Chicken Bake** is the perfect comfort meal, blending the warmth of a classic French onion soup with tender, juicy chicken. It’s easy to make and will fill your home with an irresistible aroma that will have everyone excited for dinner!