Cook lasagna noodles according to package directions. Drain and set aside.
In a large skillet, melt butter over medium heat. Add chopped onions and cook until golden brown.
In a large bowl, combine mashed potatoes, half of the shredded cheese, and half of the cooked onions. Season with salt and pepper.
In a greased 9x13 inch baking dish, layer 4 lasagna noodles, followed by half of the potato mixture. Repeat layers, ending with a final layer of noodles.
Top with remaining cheese and onions.
Cover with aluminum foil and bake for 30 minutes. Remove foil and bake for an additional 15 minutes, or until cheese is bubbly and golden.
Let stand for 10 minutes before serving. Serve with sour cream if desired.