In a mixing bowl, combine ground beef, breadcrumbs, egg, garlic, Parmesan cheese, Italian seasoning, salt, and pepper.
Mix well and shape into small meatballs.
Heat olive oil in a skillet over medium heat.
Cook meatballs until browned on all sides and fully cooked.
Add marinara sauce to the skillet and simmer for 10 minutes.
Preheat oven to 180°C (350°F).
Place sub rolls on a baking tray and fill with meatballs and sauce.
Sprinkle shredded mozzarella cheese on top.
Bake for 8–10 minutes until cheese is melted and bubbly.
Remove from oven and serve hot.