In a skillet, saute onion in butter until tender; set aside. In a bowl, combine cottage cheese, egg and onion salt. In another bowl, combine potatoes, 2/3 cup cheddar cheese, salt and pepper. Place 3 noodles in a greased 13x9-in. baking dish. Top with cottage cheese mixture and 3 more noodles. Top with potato mixture, remaining noodles and sauteed onion. Top with remaining 1/3 cup cheese. Cover and bake at 350° until heated through, 25-30 minutes. Let stand 10 minutes before serving.