Mix together onions, bread crumbs, Parmesan cheese and beef mince. Roll into 3cm 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 180 degrees C.
Arrange the bread cubes in a single layer in an ungreased 20x30cm baking dish. Mix together the cream cheese, mayonnaise, Italian herbs 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.