Preheat oven to 400 degrees. Line a baking sheet with foil, then lightly coat the foil with nonstick spray.

In a small bowl, combine the breadcrumbs 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, & pepper.

Using a wooden spoon or clean hands, stir until well combined.

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

Drizzle a small amount of mayonnaise on each meatball.

Sprinkle each meatball with mozzarella & Parmesan.

Place into oven and bake 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.

Serve immediately.