Creating Meatball Sub Sandwiches is a straightforward process. Follow these steps for a delicious outcome:

1. Preheat Oven: Preheat your oven to 375°F (190°C).
2. Mix Ingredients: In a large bowl, combine the ground beef, bread crumbs, Parmesan cheese, parsley, egg, garlic, oregano, salt, and black pepper. Mix until well combined.
3. Shape Meatballs: Using your hands, shape the mixture into meatballs, about 1 to 1.5 inches in diameter.
4. Cook Meatballs: In a large skillet, heat a little olive oil over medium heat. Add the meatballs and cook until browned on all sides, about 5-7 minutes. Remove from heat.
5. Add Sauce: Pour the marinara sauce over the meatballs in the skillet. Stir gently to coat the meatballs in sauce.
6. Transfer to Baking Dish: Place the meatballs and sauce in a baking dish.
7. Add Cheese: Sprinkle shredded mozzarella cheese over the top of the meatballs.
8. Bake: Place the baking dish in the preheated oven and bake for 15-20 minutes, or until the cheese is melted and bubbly.
9. Prepare Sub Rolls: While the meatballs are baking, slice the sub rolls in half and lightly toast them if desired.
10. Assemble Sandwiches: Once the meatballs are done, use a scoop to place meatballs and sauce into each toasted sub roll.
11. Garnish: Add any optional toppings such as sliced jalapeños, fresh basil, or additional Parmesan cheese.