From: Feelgood Eats
You can play around with the filling for this chicken. Maybe add a little spinach? Different types of cheese? I thought that a few mushrooms might be tasty in the sauce. It's really a simple and tasty recipe that you could play around with to suite your tastes, ingredients on hand, etc.
4 chicken breasts, with skin (I used skinless)
sea salt and fresh ground pepper
1 cup grated mozzarella
4 tablespoons olive oil (divided)
1/2 red onion, minced
3 tablespoons balsamic vinegar
1/4 cup chicken stock
3/4 cup heavy cream
Preheat the oven to 375.
Wash and trim the chicken breasts and use a paper towel to pat dry. Using a small knife, carefully slice a notch into the side of the chicken breast. Work slowly, moving the knife back and forth to expand the pocket. If you break through to the surface, don’t worry, it will still work out. Stuff the pocket with the grated mozzarella (or your chosen filling) and press around the open edge to help seal the pocket. Season with a generous pinch of sea salt and fresh ground pepper.
In a large oven-proof, heavy-bottomed skillet (cast-iron is ideal) heat 2 tablespoons of the olive oil and add the chicken skin-side down (I used skinless and it was fine). Cook for about five minutes; you will see the chicken cook through by about half. Transfer the chicken to the oven (without flipping) and continue to cook until it is cooked through (internal temp of 165+), about 10 minutes.
While the chicken roasts in the oven, put together the sauce. In a small saucepan add 2 tablespoons of olive oil, the minced onion and a pinch of sea salt. Stir frequently and saute until the onion softens, about 3 minutes. Add the balsamic vinegar and saute for a minute, stirring to combine. Add the chicken stock and cream and bring to a simmer, allowing all of the ingredients to mingle. Remove from the heat and pour over cooked chicken breasts. Serve immediately and enjoy!