01 - Preheat oven to 400°F. Line a baking sheet with parchment paper.
02 - In a large mixing bowl, combine ground beef, breadcrumbs, milk, egg, Parmesan cheese, parsley, garlic powder, onion powder, kosher salt, and black pepper. Mix until just blended.
03 - Form 16 golf ball-sized meatballs and arrange evenly on the prepared baking sheet. Bake for 18 to 20 minutes, until meatballs are cooked through.
04 - Transfer hot meatballs to a large bowl and toss thoroughly with barbecue sauce.
05 - Cook elbow macaroni in a large pot of salted boiling water according to package directions. Drain and set aside.
06 - In a medium saucepan, melt butter over medium heat. Whisk in flour and cook for 1 minute, stirring constantly.
07 - Gradually add milk while whisking, continue cooking until mixture thickens slightly, about 3 to 4 minutes.
08 - Remove saucepan from heat. Add shredded cheddar cheese and ground mustard, if using, and stir until cheese is melted. Season with kosher salt and black pepper.
09 - Fold cooked macaroni into the cheese sauce until evenly coated.
10 - Preheat oven broiler to high. Place split sub rolls on a baking sheet and brush their interiors with melted butter.
11 - Broil rolls for 1 to 2 minutes until lightly toasted and golden.
12 - Place 4 barbecue meatballs in each toasted roll. Spoon generous portions of mac and cheese on top.
13 - Sprinkle mozzarella cheese over each assembled sub, if desired. Return subs to broiler for 2 to 3 minutes, until cheese is bubbly and lightly browned.
14 - Finish with chopped fresh parsley as garnish, if preferred. Serve hot.