Preheat the oven to 400°F (200°C).
In a large bowl, combine the ground beef, Italian seasoned breadcrumbs, grated Parmesan cheese, whole milk, egg, minced garlic, dried oregano, dried basil, salt, and black pepper. Mix until just combined.
Shape the mixture into 16 meatballs, about 1 1/2 inches in diameter, and place them on a baking sheet lined with parchment paper.
Bake the meatballs in the preheated oven for 15 minutes, until browned and cooked through.
While the meatballs bake, heat the marinara sauce in a large skillet over medium heat.
Add the baked meatballs to the marinara sauce and simmer for 10 minutes, turning occasionally to coat evenly.
Meanwhile, heat the olive oil in a large skillet over medium heat.
Place the sub rolls cut side down in the skillet and toast for 1-2 minutes until golden and crisp.
Remove the rolls from the skillet and spoon 4 meatballs with sauce into each roll.
Top each sandwich with 2 ounces of shredded mozzarella cheese.
Place the sandwiches on a baking sheet and broil in the oven for 2-3 minutes, or until the cheese is melted and bubbly. Watch carefully to avoid burning.
Remove from the oven and serve immediately.