Line a baking sheet with foil; then lightly coat the foil with nonstick spray. In the bowl of a food processor, pulse the bread until they resemble crumbs.

Heat a small skillet over medium heat. Add the bread crumbs and toast until golden brown, about 4-5 minutes.

In a small bowl, combine the toasted bread crumbs and milk; let stand until all of the milk is absorbed.

In a large bowl, add the breadcrumb mixture, ground beef, grated onion, sour cream, egg, salt and pepper. Using a wooden spoon or clean hands, stir until well combined.

Roll the mixture into 1 1/4-to-1 1/2-inch meatballs and place on onto prepared baking sheet.

Drizzle a small amount of mayonnaise on each meatball. (A squeeze bottle or Ziploc bag is perfect for this.)

In a small bowl, combine the mozzarella and Parmesan. Sprinkle each meatball with 1 tablespoon cheese mixture.

Bake at 400 degrees until the cheese has melted and is slightly golden brown, about 15-20 minutes. Broil for an additional 2-5 minutes, or until the cheese has crisped up.