01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper.
02 - In a large bowl, combine ground beef, breadcrumbs, Parmesan, milk, egg, garlic, parsley, oregano, salt, and pepper. Mix gently until just combined, avoiding overworking the meat.
03 - Shape the mixture into 12 equal meatballs and arrange on the prepared baking sheet. Bake for 18–20 minutes until browned and cooked through to an internal temperature of 160°F.
04 - While the meatballs bake, warm the marinara sauce in a large skillet over medium-low heat. Stir in red pepper flakes if desired.
05 - Transfer the baked meatballs to the skillet, turning to coat evenly in sauce. Simmer gently for 5–7 minutes.
06 - Combine melted butter with finely grated garlic. Slice the sub rolls lengthwise without cutting all the way through. Brush the cut sides with garlic butter and toast on a baking tray for 3–4 minutes until golden.
07 - Place 3 saucy meatballs inside each toasted roll. Generously top with shredded mozzarella cheese.
08 - Return the assembled subs to the oven for 3–4 minutes until the cheese is melted and bubbly. Garnish with fresh basil or parsley if desired. Serve immediately while hot.