Preheat your Traeger grill or oven to 400°F and line a baking sheet with parchment paper.
In a large bowl, combine ground beef, breadcrumbs, Parmesan cheese, garlic, parsley, egg, oregano, basil, salt, and pepper. Mix until just combined.
Form the mixture into golf ball-sized meatballs, making about 16-20 in total.
Place the meatballs on the baking sheet or skillet and cook for 20-25 minutes in the Traeger, or 20 minutes in the oven, until they're cooked through.
While the meatballs are cooking, warm the marinara sauce in a pot over medium-low heat and add the cooked meatballs to the sauce after 10 minutes of simmering.
Slice the hoagie rolls and butter the cut sides. Toast them in a skillet, griddle, or the Traeger until golden brown.
Spoon sauce onto both sides of the rolls, place 3-4 meatballs in each roll, top with extra sauce, and sprinkle with mozzarella cheese.
Return to the Traeger or broil in the oven until the cheese is melted to your liking.
Garnish with fresh basil if desired and serve immediately.