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 until just combined, being careful not to overwork the meat.
03 - Divide the mixture into 12 equal portions and roll each into a smooth meatball. Arrange them on the prepared baking sheet with even spacing.
04 - Bake the meatballs for 18 to 20 minutes, until nicely browned on the outside and cooked through to an internal temperature of 160°F.
05 - While the meatballs bake, pour the marinara sauce into a large skillet and warm it over medium-low heat. Stir in the red pepper flakes if you want a subtle kick.
06 - Transfer the baked meatballs into the warmed marinara sauce, gently turning each one to coat evenly. Let them simmer together for 5 to 7 minutes so the flavors meld.
07 - In a small bowl, stir together the melted butter and finely grated garlic. Slice each sub roll lengthwise without cutting all the way through. Brush the cut sides generously with the garlic butter, then place them on a baking tray and toast in the oven for 3 to 4 minutes until golden.
08 - Place 3 saucy meatballs inside each toasted sub roll, spooning extra marinara over the top. Sprinkle each sub generously with shredded mozzarella cheese.
09 - Return the assembled subs to the oven for 3 to 4 minutes, just until the mozzarella is fully melted and bubbling.
10 - Finish each sub with a scattering of fresh basil or chopped parsley if desired. Serve immediately while hot and the cheese is at its stretchiest.