Mix the ground beef, egg, onion, Parm-Reg cheese, bread crumbs, Italian seasoning, Red pepper flakes, salt, pepper, garlic powder together well. Form into 12 even sized meatballs. Place in the pressure cooker. Add optional sweet onion and green pepper if using.
Combine the pasta sauce and sugar and pour over the meatballs in the pressure cooker. place the lid on the pressure cooker and cook on low pressure for 8 minutes. Allow pressure to release naturally (or release pressure, please be careful when manually releasing pressure as it will spray out hot liquid!) Remove the lid after pressure is released, skim the sauce for fat and discard.
Split the buns in half long ways (so cut them in half as you would if making a sandwhich, but split them all the way through). Lay the buns on a boiling pan and brush with oil. Place under the broiler until lightly toasted.
Remove buns from the broiler and spread 2 tablespoon of sauce on each of the toasted buns. Cut the meatballs in half and place 4 halved meatballs on the bun cut side down. Top with mozzarella cheese and return it to the broiler. Broil until the cheese is melted and bubbly. We pour the remaining sauce into the bowl and use it for dipping or adding to sandwiches. If using optional sweet onion and pepper, remove from the cooker and place in a bowl with the sauce. Serve with a salad or fries!!! Enjoy!!