Pre-heat oven to 375*. Grease a 9x13 inch baking dish with butter.
In a large skillet place butter, onion slices, salt and pepper. Sauté at medium heat until soft and brown. (About 10 to 15 minutes)
Place 4 lasagna noodles in an even layer on the bottom of the baking dish. Place, on top of noodles, 1/2 the mashed potatoes, then 1/2 the onions, then 1 cup of the sauerkraut. Repeat the layer. Top all with a layer of noodles. Bake for 30 minutes or until hot and bubbly.
Cooks note: You may substitute the sauerkraut with 2 cups of shredded Cheddar Cheese. If using shredded cheese, top the last layer of noodles with cheese. If you are short on the mashed potatoes, you may add prepared instant potatoes to have enough.