In a medium-large skillet or sauce pan heat the marinara (and red pepper flakes for heat if you wish) and meatballs together until the meatballs are heated through.
Combine melted butter, garlic powder, and fresh minced parsley together in a small dish. Slice the hoagie rolls open and place on a foil lined pan. Brush both sides of the rolls with the melted butter mixture. Place under the broiler (low setting if you have it) for 2-3 or until lightly toasted. Sprinkle rolls with shredded cheese, place back under the broiler until cheese is melted.
Place 4 meatballs on one side of each roll. Top the meatballs with sliced mozzarella. Place back under the broiler just until the mozzarella is melted. Top with any remaining marinara. Place the top roll onto the meatball side roll and slice each in half to make 8 sliders. Each slider will have 2 meatballs in it.
Serve with any remaining marinara sauce.
Tip: Double the meatballs and use a whole jar of marinara sauce and you have dinner ready for another night!
I like to use the bakery made brat buns from my local grocery store. They're big enough to hold up to the sandwich without being hard to eat like a crusty French baguette might be.
For the fresh mozzarella you can usually find a log of pre-sliced which works perfectly for these.