Pour the sauce into a large saucepan or pot over medium heat.
Add the meatballs directly to the sauce. Stir to coat, then cover and simmer on low for 20โ25 minutes, stirring occasionally, until the meatballs are heated all the way through.
While the meatballs are simmering, mix the softened butter with a shake of garlic powder and Italian seasoning. Spread onto the cut sides of the sub rolls.
Toast the rolls under the broiler for 2โ3 minutes until golden and slightly crispy. Keep an eye on them โ they go from toasted to burnt fast.
Spoon 4โ5 meatballs onto each roll and add a generous scoop of sauce.
Top with mozzarella cheese and sprinkle on some parmesan if you like.
Broil the assembled sandwiches for another 2โ3 minutes until the cheese is melted and bubbly.
Garnish with fresh basil or parsley if you're feeling fancy, and serve immediately.
A slow cooker works great here too โ dump in the meatballs and sauce on low for 4โ6 hours and they'll be perfect by dinner time. Great for a crowd.