Set Power Select at HIGH.
In greased 3-quart casserole dish, layer potatoes and onion; add water, salt and pepper.
Heat, covered, 7 to 8 minutes.
Add corned beef and cabbage; pour butter over cabbage.
Heat, covered, at HIGH 10 to 11 minutes or until cabbage is tender; stir once.
Let STAND, covered, 5 minutes before serving.