Put turkey into turkey pan and take all of the extra parts ( neck, heart, ect. ) out of the birds inside, set to the side or throw away if you don't use those for something else.
Peel the onion, and place inside the Turkey along with a couple ( 2 or 3 ) of cloves of garlic and 2 rosemary sprig. The rub the outside of the turkey with the butter all over the breast. Then salt and pepper. Place rosemary sprig on top of the turkey and several cloves of garlic in the bottom of the pan. Add the water last to the bottom of the pan. Cover the turkey with tin foil and make sure it covers the bird. ( this will self baste the turkey while it cooks ) Cook for 2 hours on 400 degrees, then turn down oven to 250 degrees for 2 more hours. An hour before you serve the turkey remove the tinfoil and brown the turkey in oven on low heat for 20 min. 200 degrees or so.