Following procedure for baking potatoes
Cook potatoes at HIGH 10 to 11 minutes, or until potatoes are almost tender.
Let stand 5 minutes; peel and slice.
Meanwhile, in 4-cup glass measure, combine butter, onion, salt and pepper.
Cook at HIGH 1 to 1 1/2 minutes, or until butter is melted.
Stir in flour; gradually add milk, stir until smooth.
Cook at MEDIUM 6 to 7 minutes, or until sauce is smooth; stir twice.
In 2-quart casserole, alternately layer potatoes and sauce forming three layers.
Cover with plastic wrap.
Cook at MEDIUM 14 to 15 minutes.
Let stand, covered, 5 minutes before serving.