Scrub and pierce potatoes; place on a microwave-safe plate. Microwave, uncovered, on high until tender, turning once, 7-9 minutes. Cool slightly. Cut a thin slice off the top of each potato and discard. Scoop out the pulp, leaving a thin shell., In a bowl, mash the pulp. Stir in the tuna, celery, onion, salad dressing, salt and pepper. Spoon into potato shells. Sprinkle with cheese. Place on a baking sheet. Broil 4-6 in. from the heat until cheese is melted, 5-6 minutes.