Mix together onions, bread crumbs, Parmesan cheese and ground beef. Roll into 1 inch diameter balls, and place in a baking pan. Bake for 15 to 20 minutes, or until beef is no longer pink. Reduce the oven temperature to 350 degrees F (175 degrees C).
Arrange the bread cubes in a single layer in an ungreased 9x13 inch baking dish. Mix together the cream cheese, mayonnaise, Italian seasoning and black pepper until smooth. Spread this mixture over each bread cube. Sprinkle with 1/2 cup of the grated mozzarella cheese.
In a large bowl, mix together spaghetti sauce, water, and garlic. Gently stir in meatballs. Pour over the bread and cheese mixture in the baking pan. Sprinkle the remaining mozzarella cheese evenly over the top.
Bake at 350 degrees F (175 degrees C) for 30 minutes, or until heated through.