Preheat oven to 350 degrees. In a large skillet, sauté the butter and chicken until chicken is browned. Remove chicken and set aside. Add onion to skillet and sauté until soft. Puree remaining ingredients in a blender then add to skillet and heat through. Place chicken in a baking dish and pour mixture on top. Bake 15 -20 minutes for boneless breasts. Bake 30-40 minutes for bone in split breasts.