Preheat the oven to 350 degrees F (175 degrees C).
Stir together the cream of potato soup, cream of mushroom soup and onion in a bowl. Heat in the microwave until hot, about 3 minutes. Make a layer of potato slices in the bottom of a 9x13 inch baking dish. Cover with a layer of the soup mixture and then sprinkle with some of the cheese. Repeat layers until ingredients are used up.
Bake for 1 hour in the preheated oven, or until potatoes are fork tender and cheese is browned on top.
