Step 1

Preheat the oven to 400 degrees and spray a baking dish with cooking spray

Step 2

Add butter to a large skillet on med to high heat. Add in sliced onions and cook for 5 minutes, stirring occasionally.

Step 3

Reduce the heat on the pan to medium and add in the garlic, pepper and thyme and cook for another minute

Step 4

Stir in the beef broth and the balsamic vinegar and cook until the onions have a jam-like consistency, about 10 minutes.

Step 5

When onions are caramelized, place about 1/3 of the mixture into the bottom of the greased baking dish.

Step 6

Place chicken on top of the onions and sprinkle with the salt.

Step 7

Place remaining onions on top of the chicken along with the mozzarella and parmesan cheeses.

Step 8

Bake for 30 minutes or until the chicken has a safe internal temperature of 165 degrees.

Step 9

Serve warm and enjoy!