Preheat your oven to 375°F (190°C).
In a mixing bowl, combine ground beef, breadcrumbs, Parmesan cheese, chopped parsley, egg, minced garlic, salt, black pepper, and Italian seasoning. Mix until well combined.
Using a meatball scoop or spoon, form the mixture into meatballs, approximately 1 to 1.5 inches in diameter. Place them on a baking sheet.
Bake the meatballs in the preheated oven for 20 minutes, or until cooked through and browned.
While the meatballs are baking, heat the marinara sauce in a saucepan over medium heat.
Once the meatballs are done, carefully transfer them to the saucepan with the marinara sauce and simmer for 5 minutes.
Cut the sub rolls in half lengthwise but not all the way through (like a hinge).
Place the meatballs with sauce into each sub roll, then top with shredded mozzarella cheese.
Place the assembled subs on a baking sheet and bake in the oven for an additional 5-7 minutes, or until the cheese is melted and bubbly.
Remove from oven, serve warm, and enjoy your delicious meatball subs!