Preheat your oven to 400°F (200°C) to prepare for baking the subs.
In a spacious mixing bowl, combine the ground beef, ground pork (if using), breadcrumbs, Parmesan cheese, parsley, minced garlic, egg, onion powder, dried Italian seasoning, and a generous pinch of salt and pepper. With clean hands or a spoon, mix the ingredients until they are thoroughly blended.
Shape the mixture into meatballs approximately the size of golf balls, aiming for about 1 inch in diameter. You should yield around 16 hearty meatballs.
Heat olive oil in a large skillet over medium heat. Once the oil is shimmering, gently add the meatballs, cooking for 5-7 minutes. Rotate them occasionally to achieve a golden-brown crust all around.
Carefully pour the marinara sauce over the partially cooked meatballs in the skillet. Cover with a lid and let them simmer for 10-15 minutes, checking occasionally, until they are fully cooked through (internal temperature should reach 160°F / 70°C).
While the meatballs are simmering, prepare the sub rolls. Slice each roll in half lengthwise but do not cut all the way through; leave a hinge to keep them intact.
Once the meatballs are cooked, use tongs to place four meatballs into each sub roll, spooning some marinara sauce over them generously.
Top each sub with a generous handful of shredded mozzarella cheese, ensuring every meatball is warmly covered.
Arrange the loaded subs on a baking sheet and place them in the preheated oven. Bake for approximately 8-10 minutes, or until the cheese is melted and reveals a delightful bubble.
Remove the baking sheet from the oven and allow the subs to cool for about a minute before serving.
Notes
Serve on a rustic wooden board with extra parsley and marinara sauce for dipping.