Preheat oven to 375 degrees F.

In a medium bowl, combine ground beef, bread crumbs, Parmesan cheese, egg, parsley, salt and pepper.

With your hands, mix well until ingredients are well combined.

Shape mixture into 12 balls.

In a large skillet over medium heat, heat 3 tablespoons of the oil.

Add the meatballs and cook until browned on both sides.

Transfer to a baking sheet and bake for 20 minutes.

Meanwhile, in a saucepan over medium heat, cook marinara sauce until warmed through.

To assemble subs, place 2 meatballs on each bun.

Top with mozzarella cheese and serve immediately.