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, being careful not to overwork the meat.
03 - Shape the mixture into 12 equal meatballs and place them on the prepared baking sheet. Bake for 18 to 20 minutes until browned and cooked through to an internal temperature of 160°F.
04 - While the meatballs bake, heat the marinara sauce in a large skillet over medium-low heat. Stir in red pepper flakes if desired.
05 - Transfer the cooked meatballs into the warm marinara sauce, turning to coat evenly. Simmer gently for 5 to 7 minutes so the flavors meld.
06 - Mix the melted butter with the 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 to 4 minutes until golden.
07 - Place 3 saucy meatballs inside each toasted roll. Spoon extra sauce over the top and sprinkle generously with shredded mozzarella.
08 - Return the assembled subs to the oven for 3 to 4 minutes, or until the mozzarella is fully melted and bubbly.
09 - Finish with a sprinkle of fresh basil or parsley. Serve immediately while hot.