Arrange potato and onions in an 8-inch square baking dish. Cover with heavy-duty plastic wrap, and vent. Microwave at HIGH 10 minutes or until potato is tender, stirring every 4 minutes. Set aside.
Coat a 2-cup glass measure with cooking spray; add margarine. Microwave at HIGH 30 seconds or until margarine melts. Add flour, salt, and pepper; stir well. Add milk, stirring until smooth. Microwave at HIGH 3 minutes or until thickened, stirring with a wire whisk every 1 1/2 minutes. Add cheese; microwave at HIGH 30 seconds or until cheese melts. Pour cheese mixture over potato and onions; stir. Microwave at HIGH 1 minute or until thoroughly heated.