Ooey Gooey Cheesy Meatballs

1/4 c. bread crumbs
1/4 c. milk
1 lb. ground beef
1 small onion, grated
1 T. sour cream
1 large egg
1/2 t. salt
1/2 t. pepper
1/4 c. mayonnaise
3/4 c. shredded mozzarella cheese
1/4 c. grated Parmesan cheese

Preheat oven to 400*.  Line a baking sheet with foil, then lightly coat the foil with nonstick spray.  In a small bowl, combine the bread crumbs and milk; let stand until all of the milk is absorbed.  In a large bowl, add the bread/crumb 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 - 1 1/2" meatballs and place onto prepared baking sheet.  Drizzle a small amount of mayo on each meatball.  In a small bowl, combine cheeses.  Sprinkle each meatball with 1 T. cheese mixture.  Place into oven and bake until cheese is melted and is slightly golden brown, about 15-20 minutes.  Broil for an additional 2-5 minutes or until cheese has crisped up.