01 - Preheat oven to 400°F and line a baking sheet with parchment paper.
02 - In a large mixing bowl, combine ground beef, ground pork, breadcrumbs, Parmesan cheese, minced garlic, parsley, egg, salt, pepper, and milk. Mix gently to incorporate; avoid overmixing.
03 - Form the mixture into 16 equal-sized meatballs. Arrange them on the prepared baking sheet.
04 - Transfer meatballs to oven and bake for 18–20 minutes until fully cooked and browned.
05 - While meatballs bake, warm marinara sauce in a large skillet over medium heat. Stir in oregano and basil.
06 - Add baked meatballs to sauce and simmer gently for 5–10 minutes, turning carefully to coat all sides.
07 - Slice each roll lengthwise without cutting through completely. Brush inside surfaces with butter and toast under broiler or in a skillet until golden brown.
08 - Place 4 meatballs coated in sauce into each roll. Sprinkle generously with shredded mozzarella cheese.
09 - Broil assembled subs for 2–3 minutes until cheese is melted and bubbly.
10 - Top with fresh basil if desired. Serve warm.