Crumble the Italian sausage into a 30-ounce casserole. Stir in the chopped onion. Micro-cook, uncovered, on 100% power for 2 1/2 to 3 1/2 minutes or till sausage is done, stirring once. Drain off fat. Stir in the catsup and oregano. Micro-cook, uncovered, on 100% power for 30 to 45 seconds or till the sausage mixture is heated through. Place the roll bottom on a paper towel-lined nonmetal plate. Spoon the sausage mixture atop the roll bottom. Top with the slice of cheese and top of the roll. Micro-cook, uncovered, on 100% of power for 30 to 45 seconds more or till cheese is melted. Serve at once.