Cook manicotti shells according to package directions, omitting salt; drain shells, and set aside. Combine soup and sour cream; stir will. Combine half of soup mixture and chicken; stir well. Set aside remaining soup mixture. Stuff manicotti shells with chicken mixture; place in a greased 12x8x2 inch baking dish. Saute onion in butter in a large skillet until tender; add mushrooms. Stir reserved soup mixture into mushroom mixture. Spoon over mannnnicotti; bake uncovered at 350 for 15 minutes. Sprinkle with cheese, and bake an additional 5 min. Yield: 4 servings. Posted to MM-Recipes Digest by "Tonya Stinson" on Dec 07, 1998,