In a large mixing bowl, combine ground beef, mild Italian sausage, chopped parsley, minced garlic, Italian breadcrumbs, egg, grated Parmesan, kosher salt, and black pepper. Use your hands to mix until just combined. Shape the meat into golf ball-sized meatballs, about 2 tablespoons each, to yield around 24 meatballs, then set them aside for cooking.

In a large saucepan, heat your favorite marinara sauce over medium heat. Gently add the formed meatballs to the sauce, ensuring they are well-covered. Cover the saucepan and simmer for 10 minutes. Then, uncover and reduce the heat to low, allowing the meatballs to cook through until they reach a safe internal temperature of 165°F, about an additional 10 minutes.

As the meatballs simmer, preheat your oven’s broiler to HIGH. Slice the hoagie rolls in half lengthwise, then lay them cut-side up on a baking sheet. Brush the insides with olive oil or melted salted butter, and generously sprinkle garlic powder over them. This step ensures your hoagie rolls will be golden and flavorful once toasted.

Once the meatballs are cooked through, remove the rolls from the oven. In each hoagie roll, place four hearty meatballs, spooning some marinara sauce over them. Then, top each sandwich with a generous sprinkle of shredded mozzarella cheese.

Slide the assembled meatball sub sandwiches back into the oven under the broiler. Keep a close eye on them as they cook, broiling for approximately 3-5 minutes, or until the cheese is bubbling and begins to brown, and the rolls are crispy.

Once the cheese is deliciously melted and the rolls are nicely toasted, remove the sandwiches from the oven. Garnish with additional chopped parsley if desired, and serve immediately while warm.