Preheat oven to 375 degrees.
In a medium bowl, combine cream cheese, spinach, Mozzarella, Parmesan and onion; blend well then spread equal portions down the center of each chicken breast half and “fold” chicken back over the filling. Place stuffed chicken in a 9- x 13-inch lightly oiled baking dish.
In a medium bowl, combine tomato sauce, basil, oregano, garlic and vinegar (if desired); pour mixture over chicken.
Cover baking dish with foil (making sure foil isn’t touching the sauce) and bake for 30 minutes or until chicken is done and juices run clear.