Preheat the oven to 400°F (200°C).
In a large mixing bowl, combine ground beef, Italian seasoned breadcrumbs, grated Parmesan cheese, whole milk, egg, minced garlic, dried oregano, dried basil, salt, and black pepper. Mix until all ingredients are evenly incorporated.
Shape the meat mixture into 16 evenly sized meatballs, about 1.5 inches in diameter.
Place the meatballs on a baking sheet lined with parchment paper or lightly greased.
Bake the meatballs in the preheated oven for 15 minutes, or until they are browned and cooked through.
While the meatballs bake, heat the olive oil in a large skillet over medium heat.
Add the baked meatballs to the skillet and pour marinara sauce over them. Simmer for 10 minutes, turning occasionally to coat the meatballs evenly.
Preheat the broiler on high.
Place the sub rolls on a baking sheet and toast them under the broiler for 1-2 minutes until lightly golden.
Divide the meatballs and marinara sauce evenly among the toasted sub rolls.
Sprinkle 2 ounces of shredded mozzarella cheese on top of the meatballs in each sandwich.
Place the assembled sandwiches under the broiler for 2-3 minutes, or until the cheese is melted and bubbly.
Remove from the oven and serve immediately.