Preheat the oven to 375°F (190°C).

In a large bowl, combine the ground beef, breadcrumbs, Parmesan, parsley, egg, garlic, Italian seasoning, salt, and pepper. Mix gently until combined, avoiding overworking the mixture.

Shape the mixture into evenly sized meatballs, about 1 1/4 inches each, and arrange them on a baking sheet lined with parchment or a lightly oiled rack.

Bake the meatballs for 25–30 minutes, turning once if you like even browning, until they reach an internal temperature of 160°F and are cooked through.

While the meatballs rest, warm the marinara sauce in a skillet over medium heat. Add the cooked meatballs to the sauce and simmer for 3–5 minutes.

Split and toast the sub rolls under the broiler or in a toaster oven until lightly crisp.

Fill each roll with 3–4 meatballs, spoon extra sauce over, and tuck a slice of provolone or mozzarella on top.

Place the assembled subs under the broiler for 1–3 minutes until the cheese is bubbly and slightly browned.