Preheat oven to 400. Thinly slice onions and place in a skillet (preferably cast iron) with butter, minced garlic and a dash of salt and pepper. Cook on low about 6-8 minutes to caramelize them then add 1/4 cup of the beef broth. Cook another 5 minutes on low.
Mix softened cream cheese, mozzarella, and onions in a bowl.
Butterfly your chicken breasts and season inside and out with salt and pepper. (I pounded mine a little thinner but you dont have to.) Stuff chicken breast with cream cheese onion mixture and use toothpicks to seal them.
Place chicken breasts in the same skillet you used to cook the onions on medium heat and sear 4 minutes on each side.
Mix remaining broth with remaining onion cream cheese mixture and place on the bottom of your baking pan and place chicken on top. Spoon some of the mixture on top the chicken breasts and bake covered 15-20 minutes until fully cooked. Remove tooth picks and serve.
