Combine butter, garlic powder and salt in a small microwave-safe dish. Heat in the microwave until melted, about 30 seconds. Alternately, heat on the stove over medium low until melted.
Line sub rolls facing up on a baking sheet (I line the baking sheet with parchment paper for easy cleanup). Use a pastry brush to spread melted butter on the inside of each sub roll. Toast on the middle oven rack for 3-4 minutes, until the insides are golden.
Meanwhile, combine pasta sauce and cooked meatballs in a medium sauce pan. Simmer over medium heat until pasta sauce is warmed through.
To assemble subs: Leave toasted subs on the baking sheet. Add 3 meatballs to each toasted sub roll. Drizzle with ¼ cup to ½ cup pasta sauce (use more if you like a saucier sub). Sprinkle each sub with 2 tablespoons shredded mozzarella.
Cook at 350 degrees for 5 minutes, or until the mozzarella is melted. Remove from oven and sprinkle with chopped parsley just before serving.
Notes
Pasta sauce: You can keep things classic with a jar of traditional spaghetti sauce or use for your favorite kind of pasta sauce. I like using roasted garlic and basil or Italian seasoned pasta sauce for extra flavor.
Make vegan: Replace shredded mozzarella with your favorite vegan shreds or by omit cheese entirely. Use vegan butter in place of regular butter and double check that your sub rolls are vegan.
This recipe yields a slightly toasty meatball sub. If you prefer a soft sub roll, just skip the initial toasting and only toast with the meatballs and cheese.
Have leftovers? These subs are best enjoyed just after cooking, but leftover subs can be stored in the fridge for 2-3 days. Reheat in at 350 degree oven for 10-15 minutes, or until completely warmed through.