French Onion Baked Chicken

Name: French Onion Baked Chicken
Prep time: 10 min
Cook time: 50 min
Total time: 1 h
Cuisine: American
Ingredients: 3 Tbsp butter; 2 tsp olive oil; 4-5 large yellow or white onions (peeled, sliced in half then cut into thin half moon shapes); 1/4 tsp black pepper; 2 sprigs fresh thyme ((or 1/2 tsp dried)); 3 cloves garlic (minced); 1 Tbsp balsamic vinegar ((or red wine or sherry)); 2/3 cup reduced sodium beef broth (divided); 4 boneless skinless chicken breasts; 1/2 tsp kosher salt; 1/2 tsp black pepper; 1/2 tsp dried thyme; 1 tsp garlic powder; 1 cup shredded mozzarella or gruyere; 1/3 cup shredded parmesan (fresh is best)