Wash and dry potatoes; pierce with a fork. Put in a white plastic grocery bag ( the kind from the check-out, not from the produce section); tie a double knot. Microwave on high 10 minutes. Bag will puff up.
Meanwhile, melt margarine in a large soup pot. Add flour; whisk to combine. Add milk slowly, whisking until lightly thickened.
Remove bag of potatoes from oven. Carefully cut open bag. It will be very hot. After potatoes are cool enough to handle, chop potatoes, including skins, and sprinkle with salt and pepper. Add to soup. Stir well and add sour cream, bacon, green onions and cheese.
Reserve some bacon, green onions and cheese for garnish.