Place the ground beef, egg, breadcrumbs, parmesan cheese, Italian seasoning, salt, and pepper into a large bowl. Use a large wooden spoon or your hands to thoroughly mix, combining all ingredients. Set aside.
Cut the mozzarella cheese into 16 cubes, roughly ½ inch x ½ inch.
Use your hands to form golf ball-sized meatballs, about 1.5 tablespoons each. This should yield about 16 meatballs.
Push 1 piece of mozzarella cheese into the center of each meatball and form the meat around the cheese.
Heat olive oil in a large cast iron skillet over medium heat.
Brown the meatballs on all sides for 6-8 minutes. Use tongs to flip the meatballs so they can cook on all sides.
Add the marinara sauce to the pan and bring to a boil. Reduce the heat to low and simmer for 10 minutes.
Serve as is, with pasta, or on a hoagie.