In a small skillet, saute onion and bell pepper with the italian dressing until slightly softened. Toss in the roast beef to heat throughout.
In the meantime, heat oven to about 400 degrees. Split the sub buns and smear butter on all four halves. Place in oven until it starts to brown a little.
Remove from oven and add cheese to each half, so that there is cheese on top and bottom of sandwich. Add the meat and pepper/onion mixture to the bread and pop back into the oven just to melt the cheese.
Remove from oven and top with banana pepper rings. Put the top on and enjoy while hot!