Cook manicotti shells in boiling salted water until tender, 15 to 20 minutes; drain. Combine water and instant minced onion; let stand 5 minutes.
Combine cream cheese and sour cream sauce mix; blend in milk. Stir in softened onion, mushrooms, parsley, salt and pepper. Add turkey or chicken; mix well. Spoon mixture into cooked manicotti shells. Arrange stuffed manicotti in a single layer in a 10"x6"x2" baking dish.
Prepare cheese sauce mix according to package directions. Pour sauce over manicotti. Sprinkle with Parmesan cheese. Bake, covered at 350 degrees until heated through, 35 to 40 minutes. Makes 6 servings.