In a 8inch skillet,cook onion and sweet pepper in salad dressing until tender. Remove skillet from heat.
For each sandwich, place the bottom of a roll on a 12 inch square of foil. Top with turkey pastrami, cheese, and tomato slices. Spoon on the onion-sweet pepper mixture, including any remaining dressing. Spread cut side of the roll top with mustard; place atop the fillings. Wrap each sandwich in the foil, sealing ends.
Heat sandwiches in a 350 oven for 15 to 20 minutes or until cheese is melted. Serve warm.