Ingredients:

• 1 1/2 – 2 lbs chicken breasts, boneless and skinless (pounded down to even about 1″ thick)
• 2 teaspoons garlic powder
• 1 teaspoon dried oregano
• salt and fresh cracked pepper
• 1 cup shredded Swiss cheese (can substitute Mozzarella)
• 2 tablespoons butter
• 5 large sweet onions, thinly sliced
• 3 cloves garlic, minced
• 1/4 to 1/2 cup balsamic vinegar
• parsley, for garnish if desired

Directions:

1. Preheat oven to 375°. Lightly grease a 9×12-inch casserole pan and set aside.
2. Melt butter in a large skillet over medium-high heat. Add onions and season with salt and pepper. Cook for 5-8 minutes until softened.
3. Reduce heat to medium-low and cook, stirring occasionally, until onions are caramelized – about 15-20 minutes.
4. Pour in 1/4 cup – 1/2 cup of balsamic vinegar (or more) to deglaze if your pan gets too dry, and keep stirring until onions are well browned.
5. Stir in fresh minced garlic and cook until fragrant, about 1 minute. Turn off the heat and allow to cool slightly.
6. Place chicken breasts in pan and season chicken with oregano, salt, pepper, and garlic powder. Go easy on the salt because the onions, cheese, and vinegar are salty already.
7. Place some of the  caramelized onions on top of each chicken breast. Sprinkle shredded cheese on top and bake for 20-25 minutes in the oven (or more, depending on the thickness of your chicken breasts), until fully cooked and cheese is bubbly.
8. Before  removing from oven broil for 2 minutes to crisp up the cheese. Garnish  with parsley, if desired, and serve immediately.