1. Heat oven to 350 degrees F. Rub taco seasoning under and over skin of turkey.
2. Peel and quarter onion. Place 2 quarters in cavity and 2 under loose flap of skin near wings. Tie legs, then transfer turkey to a rack in a large roasting pan. Pour in water. Bake at 350 degrees for 2 3/4 hours.
3. Gravy. Remove turkey from pan to platter. Pour pan juices into fat separator. Skim off fat, returning 3 tablespoons to pan. Over medium heat, whisk in flour. Carefully whisk in 1/2 cup strained pan juices and 1 1/2 cups low-sodium chicken broth. Cook 3 minutes, until thickened.