01 - Preheat the oven to 400°F. Line a rimmed baking sheet with parchment paper.
02 - In a large mixing bowl, combine the ground beef, breadcrumbs, Parmesan cheese, milk, egg, minced garlic, parsley, oregano, salt, and pepper. Mix gently with your hands until just combined, being careful not to overwork the meat.
03 - Form the mixture into 12 evenly sized meatballs and arrange them on the prepared baking sheet. Bake for 18–20 minutes until browned on the outside and cooked through to an internal temperature of 160°F.
04 - While the meatballs bake, pour the marinara sauce into a large skillet set over medium-low heat. Stir in the red pepper flakes if using. Bring to a gentle simmer.
05 - Transfer the baked meatballs into the simmering sauce, turning to coat evenly. Let them simmer gently for 5–7 minutes so the flavors meld together.
06 - In a small bowl, stir together the melted butter and grated garlic. Slice each sub roll lengthwise without cutting all the way through. Brush the cut sides generously with the garlic butter and place on a baking tray. Toast in the oven for 3–4 minutes until golden and crisp.
07 - Place 3 saucy meatballs into each toasted roll. Spoon extra marinara over the top and sprinkle generously with shredded mozzarella cheese.
08 - Return the assembled subs to the oven for 3–4 minutes, or until the mozzarella is fully melted, bubbly, and lightly golden in spots.
09 - Finish each sub with a sprinkle of fresh basil or parsley if desired. Serve immediately while hot.