Heat a large skillet over medium to medium-high heat.

Add the butter, olive oil, and sliced onions. Season with black pepper and add the fresh thyme.

Cook for 8 minutes while stirring occasionally until the onions start to soften.

Reduce the heat to medium to medium-low and continue cooking for an additional 15 minutes.

Stir occasionally until the onions become caramelized and golden. Add the minced garlic and cook for 1 minute.

Stir in the balsamic vinegar and ⅓ cup of beef broth, mixing until the mixture becomes thick and jam-like.

Preheat your oven to 400°F.

Butter a 9x13-inch baking dish. Add about ½ cup of the caramelized onions to the baking dish along with the remaining ⅓ cup of beef broth. Mix well to combine.

Season the boneless skinless chicken breasts with kosher salt, black pepper, dried thyme, and garlic powder.

Place the chicken in the baking dish and top each breast with the caramelized onions and cheese.

Bake in the preheated oven for 25 minutes or until the chicken reaches an internal temperature of 165°F.

Ensure the cheese is melted and bubbly, creating a delicious crust on top.