Preheat the Oven: Preheat your oven to 375°F (190°C).
Make the Meatballs: In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, egg, minced garlic, oregano, basil, red pepper flakes, salt, and pepper. Mix until just combined, being careful not to overmix.
Form the Meatballs: Roll the mixture into meatballs, about 1.5 inches in diameter, and place them on a baking sheet lined with parchment paper.
Bake the Meatballs: Bake the meatballs in the preheated oven for 20-25 minutes, or until they are cooked through and golden brown.
Heat the Marinara: While the meatballs are baking, pour the marinara sauce into a medium saucepan and heat over medium heat until warm.
Assemble the Subs: Once the meatballs are done, remove them from the oven and carefully place 3-4 meatballs into each sub roll. Spoon some warm marinara sauce over the meatballs.
Add Cheese: Generously sprinkle shredded mozzarella cheese on top of the sauce-covered meatballs.
Melt the Cheese: Place the assembled subs back on a baking sheet and return them to the oven for an additional 5-7 minutes, or until the cheese is melted and bubbly.
Serve: Remove from the oven, garnish with fresh basil leaves if desired, and serve hot!