Make lasagna first. Boil for 8 minutes (slightly hard); rinse with cold water, drain and let sit. Make mashed potatoes; saute 5 onions and 1 to 21 1/2 cups margarine on low heat until onions are soft. Grat e Cheddar cheese and add to potatoes; add onions and butter mixture. (Leave a little out to spread on top when through layering.) In 9 x 13-inch pan, layer butter on bottom of pan, then lasagna noodles, then potato mixture. After 3 layers (lasagna on top), spread remaining butter and onions on top.
Cover with foil and bake at 350 degrees for about 30 to 40 minutes.
Note: The layers of the potato mix should not be more than 1/2-inch deep.