You can use leftover mashed potatoes in this simple pierogi casserole dish. Just make sure you warm them up first.
Cook lasagna noodles according to package directions. Drain and set aside.
Preheat oven to 375 degrees. Spray a 13"x 9" baking dish (3 quart) with cooking spray.
Melt butter in large skillet over medium heat. Saute onions with a little salt and pepper until soft and lightly browned, about 8 minutes. Stir onion mixture and 1/2 the cheese into the mashed potatoes.
Spread 4 of the lasagna noodles on the bottom of casserole dish and top with 1/2 of the potato mixture. Repeat with another layer of lasagna noodles and mashed potatoes mixture and place final 4 lasagna noodles on top. Sprinkle reserved cheese on top.
Cover baking dish with foil and bake 30-35 minutes or until heated through.
For added flavor, top with bacon bits and sour cream.