Cook shells without salt; drain.
Combine soup and sour cream, mix well. Combine 1/2 soup mixture and chicken; mix well. Reserve remaining soup mixture.
Stuff shells with chicken mixture. Place in greased 12x8x2-inch baking dish.
Sauté onion and mushrooms in butter in large skillet until tender; set aside. Combine water and bouillon, stirring until dissolved. Add to reserved soup mixture, mix well. Stir soup mixture into mushroom mixture. Spoon over manicotti.
Bake, uncovered, at 350 degrees for 15 minutes. Sprinkle with cheese and bake an additional 5 minutes. Makes 4 to 6 servings.