Crumble the Italian sausage into a 1 litre casserole. Stir in the chopped onion. Micro-cook, uncovered, on 100% power for 3.5 to 4.5 minutes or till sausage is done, stirring once. Pour off fat. Stir in catsup and oregano. Micro-cook, uncovered, on 100% power for 1 to 1.5 minutes or till sausage mixture is heated through. Place roll bottoms on a paper towel-lined plate. Spoon some sausage mixture atop each roll bottom. Top each with a slice of cheese and the top of the roll. Micro-cook, uncovered, on 100% power for 45 seconds to 1 minute more, or until cheese is just melted.