Keto French Onion Chicken Bake

Contents:
4 tablespoons of Butter
2 tablespoons Balsamic Vinegar
2/3 c Low Sodium Broth
4 Onions, peeled and sliced
1 c. Mozzarella Cheese, chopped
4 Chicken Breasts, boneless, skinless
1/2 c Parmesan Cheese, grated
1/2 teaspoon Pepper
1/2 teaspoon salt
4 cloves of garlic, minced
1 teaspoon of thyme