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, parsley, oregano, salt, and pepper. Gently fold the ingredients together until evenly incorporated, being careful not to overmix.
03 - Form the mixture into 12 uniform 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, pour the marinara sauce into a large skillet and warm over medium-low heat. Add the red pepper flakes if using and stir to combine.
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 grated garlic. Slice each sub roll lengthwise without cutting all the way through. Brush the cut sides generously with the garlic butter mixture. Place the rolls on a baking tray and toast in the oven for 3 to 4 minutes until golden.
07 - Place 3 saucy meatballs into each toasted roll. Spoon extra marinara sauce over the top, then sprinkle each sub generously with shredded mozzarella cheese.
08 - Return the assembled subs to the oven for 3 to 4 minutes until the mozzarella is fully melted, bubbly, and lightly golden in spots.
09 - Remove from the oven and garnish with fresh chopped basil or parsley if desired. Serve immediately while hot.