Preheat the oven to 200°C/400°F/Gas 6. Spray a large, rimmed cookie sheet with nonstick cooking spray and set aside*.

Add the beef, pork, onion, garlic, Parmesan cheese, breadcrumbs, eggs, milk, Italian seasoning, and crushed red pepper flakes, if using them, to a large mixing bowl. Season with salt and black pepper, to taste.

Combine thoroughly with clean hands until all ingredients are thoroughly incorporated into the mixture. Do not overwork.

Transfer the mixture to a clean work surface and spread it into a uniformly thick rectangle and cut it into 12 equal-sized portions, as shown.

Divide the cheese sticks evenly between the 12 squares and form the meat around the cheese firmly with your hands to create the meatballs.**

Place the finished meatballs on the prepared baking sheet. Repeat with remaining sections until complete.

.Place the baking sheet on the top oven rack for 10 minutes. Remove from the oven and carefully turn each meatball with a spatula. Return to the oven and bake for another 8-10 minutes, or until an instant-read thermometer reads 160°F/71°C when inserted into the thickest part of the meatballs.***

Remove the baking sheet from the oven and set it aside, but do not turn off the oven.

Use paper towels to remove excess grease from the meatballs.

Pour the marinara sauce into a large skillet set over medium-low heat and carefully add the meatballs. Simmer for 4-5 minutes, or until the sauce is heated through, while carefully turning the meatballs to coat in the sauce.

Carefully slice open the hoagie buns and arrange them on a lined baking sheet. Fill each bun with two or three meatballs and some of the sauce. Top each sub with two or three slices of Provolone cheese and place in the oven for 3-5 minutes until the cheese is melted.

Remove the meatball subs from the oven and serve immediately with your choice of sides. Enjoy!