• Caramelize the Onions:

Melt the butter in a large skillet over medium heat. Add the onions and cook, stirring occasionally, for about 20-25 minutes until they are golden and caramelized. Add the garlic and cook for an additional 2 minutes.

• Prepare the Chicken:

While the onions are cooking, season the chicken breasts with salt and pepper. In a separate oven-safe skillet or pan, sear the chicken on both sides until golden brown, about 4-5 minutes per side. Remove and set aside.

• Make the Sauce:

Stir the flour into the caramelized onions and cook for 1 minute. Slowly add the beef broth, stirring constantly to avoid lumps. Add the balsamic vinegar and let the mixture simmer for 5 minutes until thickened.

• Combine and Bake:

Preheat the oven to 375°F (190°C). Place the chicken back into the skillet, spoon the onion sauce over the chicken, and sprinkle with shredded Gruyère or Swiss cheese. Transfer the skillet to the oven and bake for 20 minutes or until the chicken is cooked through and the cheese is melted and bubbly.

• Garnish and Serve

Once done, garnish with fresh thyme and serve the French Onion Soup Chicken with mashed potatoes, crusty bread, or a green salad for a complete meal.