Preheat your oven to 375°F (190°C). This ensures that the chicken cooks evenly and thoroughly.

Season the chicken breasts with salt and pepper on both sides. This step enhances the flavor of the chicken.

In a large skillet, heat the olive oil over medium-high heat. Once hot, add the chicken breasts and sear for 5-6 minutes on each side until golden brown. This locks in the juices.

Remove the chicken from the skillet and set aside. Do not clean the skillet; you will use the fond (brown bits) for the onions.

In the same skillet, add the butter and let it melt over medium heat. Add the sliced onions.

Sprinkle the sugar over the onions. This helps them caramelize faster by drawing out moisture.

Cook the onions, stirring occasionally, for about 15-20 minutes until they are golden brown and soft. If they start to stick, add a splash of beef broth to deglaze the pan.

Once caramelized, add thyme and the remaining beef broth. Stir well and let simmer for another 5 minutes to combine flavors.

In an oven-safe baking dish, place the seared chicken breasts. Spoon the caramelized onions evenly over each piece.

Sprinkle the gruyère and mozzarella cheese generously over the onions and chicken. This will create a delicious, gooey topping.

Transfer the baking dish to the preheated oven and bake for 20-25 minutes, or until the chicken is cooked through (internal temperature should reach 165°F or 74°C) and the cheese is bubbly and golden.

Remove from the oven and let it rest for 5 minutes. This allows the juices to redistribute.

Garnish with chopped fresh parsley before serving. This adds a pop of color and freshness.