In a large mixing bowl, combine the ground beef or Italian sausage, breadcrumbs, grated Parmesan cheese, egg, dried oregano, dried basil, garlic powder, salt, and pepper. Mix until well combined.
Shape the Meatballs: Roll the mixture into meatballs, about 1 to 1.5 inches in diameter, and place them in the slow cooker.
Pour the marinara sauce over the meatballs in the slow cooker, ensuring they are all covered.
Cover and cook on low for 6-8 hours or on high for 3-4 hours, until the meatballs are cooked through and tender.
Prepare the Subs:
- About 15-20 minutes before serving, preheat your oven to broil. Slice the hoagie rolls lengthwise, leaving one side intact.
- Place the rolls open-faced on a baking sheet and lightly toast them under the broiler for 1-2 minutes, until just golden brown.
Assemble the Subs: Using a slotted spoon, place meatballs and some sauce onto each toasted hoagie roll. Sprinkle with shredded mozzarella cheese.
Return the assembled meatball subs to the oven under the broiler for another 1-2 minutes, or until the cheese is melted and bubbly.
Garnish and Serve: Remove from the oven and garnish with chopped fresh parsley, if desired. Serve hot and enjoy your delicious Slow Cooker Meatball Subs!