In a large mixing bowl, combine the ground beef, Italian sausage, breadcrumbs, Parmesan cheese, parsley, garlic, egg, salt, pepper, and Italian seasoning. Mix until well combined and shape into meatballs.
In a large skillet over medium heat, brown the meatballs in batches for 4-5 minutes until nicely browned on all sides. Transfer browned meatballs to a plate and set aside.
In a 9x13 inch baking dish, place the sub rolls cut side up and spoon a little marinara sauce onto each roll.
Place the browned meatballs on top of the sauced sub rolls, sprinkling them evenly throughout the dish.
Pour the remaining marinara sauce over the meatballs, ensuring they are all generously coated.
Sprinkle shredded mozzarella cheese over the top, and if desired, toss on some sliced black olives.
Cover the dish with aluminum foil and bake in the preheated oven for 20 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden brown.
Remove from the oven and let cool for a few minutes. Garnish with chopped fresh basil if desired, slice, and serve warm.
Notes
Optional olives and basil can be added for extra flavor.