Preheat your grill to 350F.
Place the sausages over direct heat on your grill, and cook until fully cooked - a few minutes per side. When they're done, move them to a cooler spot on your grill to keep them warm, or move them to a plate and set aside. If using hot dogs, cook them on the cooler part of the grill, so they can grill all the way through, then move them to the hot side to char to your taste.
While the sausages cook, place a cast iron skillet on your grill over direct heat (or use the top burner of your grill has one).
Melt the butter, or heat up the olive oil, then add in the onions and and green bell peppers and saute on high until lightly browned.
Add in the tomato and cook for one minute, then finally add the thawed corn and peas, and toss. Season with salt and pepper.
Place the cooked sausages in the buns, top with the veggies, a sprinkle of Parmesan cheese, and string potatoes. Dress with mayo, ketchup and mustard, if desired!