Peel potatoes and onion. Grease microwaveable dish with butter. Slice potatoes about 1/8 inch thick and place in dish. Cover bottom only. Sprinkle this with season salt. Slice onion about 1/8 inch thick and layer over potatoes. Add second layer of potatoes. Sprinkle with season salt. Then layer of onions. Repeat potatoes, season salt, onions and dot top with butter. Cover and microwave on high 3-4 minutes for each potato used in covered pan.