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Preheat the oven to 400 degrees and spray a baking dish with cooking spray
Add butter to a large skillet on med to high heat. Add in sliced onions and cook for 5 minutes, stirring occasionally.
Reduce the heat on the pan to medium and add in the garlic, pepper and thyme and cook for another minute
Stir in the beef broth and the balsamic vinegar and cook until the onions have a jam-like consistency, about 10 minutes.
When onions are caramelized, place about 1/3 of the mixture into the bottom of the greased baking dish.
Place chicken on top of the onions and sprinkle with the salt.
Place remaining onions on top of the chicken along with the mozzarella and parmesan cheeses.
Bake for 30 minutes or until the chicken has a safe internal temperature of 165 degrees.
Serve warm and enjoy!