1. Preheat oven to 350 degrees F. Cook noodles, drain.

2. While noodles are cooking, melt butter and sauté the onions until tender.

3. Place half of the cooked noodles in the bottom of a 13×9 casserole dish.

4. Layer with 6 slices of cheese, all the cooked mashed potatoes, the remaining noodles, then top with remaining 6 slices of cheese.

5. Poke holes through the top layer of cheese; pour butter and onion mixture over top evenly so it goes through the layers.

6. Bake for 15 minutes, or until cheese is lightly browned. Serves 6.