Prepare stuffing according to package directions, and set aside in a large bowl.
Preheat oven to 350 degrees F (175 degrees C).
Bring water to boil in a medium saucepan over medium heat, and stir in the onion, celery and sage. Boil 10 minutes, or until the onion is soft. Stir into the prepared stuffing.
Rinse turkey, and pat dry. Loosely fill the body and neck cavities with the stuffing mixture.
Place turkey in a large roasting pan and cook 3 to 3 1/2 hours in the preheated oven, or until the internal temperature of the thigh meat is 180 degrees F (80 degrees C) and the stuffing is at least 165 degrees F (75 degrees C).