Place the ground beef, breadcrumbs, egg, Italian seasoning, salt, and black pepper in a mixing bowl. Mix with your hands until the ingredients are evenly combined.
Shape the mixture into 8 meatballs, each about 2 inches wide.
Heat a large skillet over medium heat. Add the meatballs and cook for 8 to 10 minutes, turning them every few minutes until browned on all sides.
Pour the marinara sauce into the skillet. Reduce the heat to low, cover, and simmer for 10 minutes so the meatballs finish cooking and soak up the sauce.
While the meatballs simmer, split the slider buns and place the bottom halves on a baking sheet. Put one meatball on each bun bottom.
Top each meatball with half a slice of mozzarella. Broil for 1 to 2 minutes until the cheese melts.
Add the top buns, sprinkle with chopped parsley, and serve right away.