Preheat your oven to 400°F (200°C). This ensures the meatballs cook evenly and develop a nice crust.

In a large mixing bowl, combine ground beef, breadcrumbs, parmesan cheese, egg, minced garlic, parsley, salt, black pepper, and red pepper flakes.

Mix gently with your hands until just combined. Overmixing can lead to tough meatballs.

Form the mixture into 1.5-inch meatballs and place them on a baking sheet lined with parchment paper.

Bake in the preheated oven for 20-25 minutes, or until they reach an internal temperature of 160°F (71°C).

While the meatballs are baking, heat olive oil in a saucepan over medium heat.

Add the chopped onion and sauté for about 5 minutes, or until translucent. This step builds the flavor base for your sauce.

Stir in the minced garlic and cook for an additional minute until fragrant.

Add the crushed tomatoes, oregano, basil, sugar, salt, and black pepper. Stir to combine.

Bring the sauce to a simmer and let it cook for about 15 minutes, stirring occasionally. This allows the flavors to meld.

Once the meatballs are cooked, add them to the marinara sauce and let them simmer together for 5 minutes.

Slice the sub rolls open and place them on a baking sheet.

Spoon 3-4 meatballs along with sauce into each roll.

Top each sub with shredded mozzarella cheese.

Place the assembled subs in the oven for an additional 5-10 minutes, or until the cheese is melted and bubbly.

Remove from the oven, garnish with fresh basil if desired, and serve hot.