Prepare the Meatballs: In a large bowl, combine the breadcrumbs and milk. Let soak for 5 minutes. Add the ground beef, ground pork, egg, Parmesan cheese, garlic, parsley, oregano, basil, red pepper flakes (if using), salt, and pepper. Gently mix until just combined. Do not overmix.

Form the Meatballs: Roll the meat mixture into 1 1/2-inch meatballs.

Cook the Meatballs: Choose either baking or pan-frying:

Baking: Preheat oven to 400°F (200°C). Place the meatballs on a baking sheet lined with parchment paper. Bake for 20-25 minutes, or until cooked through and lightly browned.

Pan Frying: Heat 2 tablespoons of olive oil in a large skillet over medium heat. Brown the meatballs on all sides, then reduce heat to low, cover, and cook for another 10-15 minutes, or until cooked through.

Prepare the Sauce: While the meatballs are cooking, heat the olive oil in a large saucepan over medium heat. Add the onion and cook until softened, about 5 minutes. Add the garlic and cook for another minute until fragrant. Stir in the crushed tomatoes, tomato sauce, oregano, basil, sugar (if using), salt, and pepper. Bring to a simmer, then reduce heat to low and simmer for at least 30 minutes, or up to 1 hour, stirring occasionally.

Assemble the Subs: Preheat broiler. Cut the hoagie rolls in half lengthwise. Place the cooked meatballs in the sauce and simmer for a few minutes to coat them evenly. Arrange the meatballs in the hoagie rolls. Spoon some of the sauce over the meatballs. Top with shredded mozzarella cheese.

Broil: Place the subs on a baking sheet and broil for 2-3 minutes, or until the cheese is melted and bubbly.

Serve: Garnish with fresh basil leaves (if using) and serve immediately.