In a medium bowl, combine the spinach, Parmesan cheese and onion and mix thoroughly. Stuff the cavity of each chicken breast with 1/4 of the mixture. (NOTE: Bone-in split breasts usually have a little cavity; if one is not naturally present, use a sharp knife to make a slit in the side of the breast.)
Lay stuffed breasts in a 9x13 inch baking dish, and cover with the sauce.
Bake at 350 degrees F (175 degrees C) for about 35 minutes, or until chicken is cooked through and tender; sprinkle with mozzarella cheese and bake another 5 minutes, until cheese is melted.
