Preheat oven to 350 degrees. Melt butter in a large cast iron or oven-safe skillet over medium heat, then add onions. Allow to cook down about three minutes.
Meanwhile, prep the chicken breast. If necessary, butterfly each breast so that no piece is more than 1 ½ inches thick. Pat it dry and season all sides with sea salt, black pepper, cayenne, and garlic powder.
Once onion begins to soften and become translucent, add chicken breasts to pan. Cook each side for three minutes, spooning butter and caramelized onions over top occasionally.
Remove skillet from stove top and place in oven. Bake for about 17 minutes or until cooked through. Internal temperature should reach 165 degrees Fahrenheit.