Place roast beef in bowl with Worcestershire sauce for 15 to 20 minutes. Sauté onions, bell peppers in vegetable oil. Add the meat to onions and bell peppers and allow to cook for another 5 minutes. Toast bread in oven until crisp. Place meat and vegetable mixture on top of the bread, cover with Swiss cheese. Broil until cheese has melted. Add mayonnaise and serve with leftover juices.