Crush 2 cups of potato chips; set aside. Whisk together soup and milk; set aside. Place half of onion slices in the bottom of a 13"x9" baking pan coated with non-stick vegetable spray. Spread uncrushed chips over onions; add one cup cheese and half of soup mixture. Repeat layering. Top with reserved crushed chips. Bake, uncovered, at 350 degrees for one hour.