Step 1

Preheat oven to 400 degrees F (200 degrees C).

Step 2

Mix ground beef, green onion, bread crumbs, and Parmesan cheese together in a bowl; form into 1-inch meatballs. Arrange meatballs in a baking sheet.

Step 3

Bake in the preheated oven until no longer pink in the center, 15 to 20 minutes. An instant-read thermometer inserted into the center should read at least 160 degrees F (70 degrees C).

Step 4

Arrange bread slices in a single layer in a 9x13-inch baking dish. Combine cream cheese, mayonnaise, Italian seasoning, and pepper in a bowl; spread over bread slices. Sprinkle 1/2 of the mozzarella cheese over the cream cheese mixture.

Step 5

Combine spaghetti sauce, water, and garlic in a bowl; add meatballs. Pour spaghetti sauce mixture over bread. Sprinkle remaining mozzarella cheese over casserole.

Step 6

Bake in the preheated oven until cheese is melted and bubbling, about 30 minutes.