1Preheat oven to 350 degrees F. Coat a 9- x 13-inch baking dish with cooking spray.
Cook lasagna noodles according to package directions; drain and set aside.
Meanwhile, in a large skillet, heat oil over medium-high heat. Saute onions 8 to 10 minutes, or until lightly browned; reserve 1/2 cup sauteed onions for topping. Add remaining onions, pepper, and cheese to mashed potatoes; mix well.
Place 3 noodles on bottom of prepared baking dish. Spread 1/3 of potato mixture over noodles. Repeat layers until a total of 4 layers of noodles and 3 layers of potato mixture have been placed (with a layer of noodles on top). Top with remaining sauteed onions.
Cover with aluminum foil and bake 25 to 30 minutes, or until heated through.