Stuffing Topped Chicken

Ingredients: 4 boneless, skinless chicken breasts 2 large onions, thinly sliced 2 tablespoons olive oil 2 tablespoons butter 1 cup beef broth 1 tablespoon balsamic vinegar (optional) 1 teaspoon dried thyme 1 cup shredded mozzarella or Gruyère cheese 1/2 cup shredded Parmesan cheese Salt a