Heat the oven to 375°F. Lightly grease 8 cups of a standard muffin pan.
In a large mixing bowl, combine the ground beef, egg, breadcrumbs, Parmesan, onion, garlic, Italian seasoning, salt, black pepper, and 2 tablespoons of the marinara sauce. Mix gently until evenly combined. Avoid packing the mixture tightly.
Divide the beef mixture among the 8 prepared muffin cups. Press it in lightly, leaving the tops slightly rounded.
Bake for 20 minutes. Spoon the remaining marinara sauce over the muffins, then sprinkle the mozzarella evenly on top.
Return the pan to the oven and bake for about 5 minutes more, until the cheese is melted and the centers are cooked through. Let the muffins rest in the pan for 5 minutes before serving.