Preheat your oven to 375°F (190°C).

In a large skillet, heat 2 tablespoons of olive oil over medium heat. Add the thinly sliced onions and sauté for 15–20 minutes until golden brown and caramelized.

Stir in 3 minced garlic cloves and cook for an additional 1–2 minutes.

Pour in 2 tablespoons of balsamic vinegar and 1 cup of beef broth. Add 1 tablespoon of Dijon mustard and 1 teaspoon of thyme, and simmer for 5 minutes.

Season 4 boneless, skinless chicken breasts with salt and pepper, then place them in a baking dish.

Spoon the caramelized onion and sauce mixture over the chicken breasts.

Sprinkle 1 cup of shredded Gruyère cheese on top.

Bake for 25–30 minutes until the chicken is cooked through and the cheese is bubbly and golden brown.

Allow the chicken to rest for 5 minutes before serving and garnish with freshly chopped parsley.