01 - Preheat the oven to 400°F. Line a baking sheet with parchment paper and set aside.
02 - In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, milk, egg, minced garlic, chopped parsley, dried oregano, salt, and black pepper. Mix gently with your hands or a fork until just combined, being careful not to overwork the meat.
03 - Shape the mixture into 12 equal-sized meatballs and arrange them on the prepared baking sheet. Bake for 18 to 20 minutes until golden brown 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 the red pepper flakes if using. Keep the sauce at a gentle simmer.
05 - Transfer the baked meatballs into the simmering marinara sauce, turning to coat evenly. Let them simmer gently for 5 to 7 minutes so the flavors meld together.
06 - Stir together the melted butter and finely grated garlic to make garlic butter. Slice each sub roll lengthwise without cutting all the way through. Brush the cut sides generously with garlic butter and place on a baking tray. Toast in the oven for 3 to 4 minutes until golden and crisp.
07 - Place 3 saucy meatballs inside each toasted sub roll, spooning extra marinara over the top. Sprinkle each sub generously with shredded mozzarella cheese.
08 - Return the assembled subs to the oven for 3 to 4 minutes, or until the mozzarella is fully melted, bubbly, and lightly golden in spots.
09 - Remove from the oven and garnish with chopped fresh basil or parsley if desired. Serve immediately while hot.