In a medium-to-large high-sided skillet, sauté onions in olive oil over medium heat until soft, about 5-6 minutes. Season with salt and pepper, then add the garlic.

Sauté for another 2-3 minutes

Add the marinara sauce, red pepper flakes, fresh basil, and meatballs, then simmer over low-medium heat for 5 minutes.

Slice each hoagie roll down the middle. Do not cut all the way through.

Brush each roll with some softened butter.

Arrange the rolls on a baking sheet and place into the preheated oven for 3-4 minutes.

Assemble the sub sandwich. Fill each roll with 4-5 meatballs and top with sauce mixture.

Top each roll with grated mozzarella or provolone cheese.

Turn oven to broil and return the hoagies to the bottom shelf of the oven until the cheese has melted. This will take 1-2 minutes.

Sprinkle with parsley and serve.