In a small bowl slightly melt 1/3 cup margarine. Add parmesan bit by bit until the mixture is not runny. Set aside. You might not need all of the cheese.
Cut both peppers in half and remove core and seeds. Slice and then cut slices in half.
Stir fry peppers and onions in margarine until onions are fried to your liking. Set aside.
Fry roast beef just until heated through.
Spread parmesan mixture on one side of each piece of bread.
In a frying pan, place two slices of bread spread side down. place one slice of cheese on one piece of bread and a half slice on the other. If you like a cheesy sandwich put one whole slice on each bread slice.
Place roast beef, onions, and peppers on one piece of bread. Top with other slice of bread.
Flip sandwich onto a plate and serve.
If this is for an appetizer, cut sandwiches in fourths and stick a toothpick through it to hold it together.