Mix together onions, breadcrumbs, Parmesan cheese and minced beef. Roll into 2.5cm diameter balls and place on a baking tray. Bake for 15 to 20 minutes or until beef is no longer pink. Reduce the oven temperature to 180 C / Gas 4.
Arrange the bread cubes in a single layer on an ungreased 20x30cm baking dish. Mix together the cream cheese, mayonnaise, dried Italian herbs and black pepper until smooth. Spread this mixture over each bread cube. Sprinkle with 1/4 of the grated mozzarella cheese.
In a large bowl, mix together pasta sauce, water and garlic. Gently stir in meatballs. Pour over the bread and cheese mixture in the baking dish. Sprinkle the remaining mozzarella cheese evenly over the top.
Bake at 180 C / Gas 4 for 30 minutes or until heated through.