In a large mixing bowl, combine ground beef, breadcrumbs, egg, salt and pepper, minced onion, garlic cloves, and chopped parsley.
Shape into small balls (about 1 1/2 inches in diameter).
Place meatballs on a parchment-lined baking sheet.
Broil for 7 to 10 minutes.
Set aside until cool enough to handle.
Preheat oven to 400 degrees.
Flour your work surface.
Place puff pastry squares onto surface.
Add a heaping tablespoon of shredded mozzarella onto center of pastry square.
Place meatball on top of the cheese.
Wrap puff pastry around meatball.
Place on parchment-lined baking sheet, seam side down.
Brush with egg wash.
Bake for 25 minutes.
Serve atop tomato sauce, and garnish with basil and Parmesan.