While turkey roasts, place giblets and neck in heavy 1 1/2-quart saucepan. Cover with cold water. Bring to a boil over high heat, skimming scum from the top.
Add vegetables and, if needed, a little additional water to cover them; return to a boil.
Lower heat to a simmer; cook slowly for about 2 hours, until liquid has reduced to a rich stock. Strain, pressing stock from vegetables; discard vegetables. Reserve stock; you should have about 1 1/4 cups.
Chop giblets in small cubes; reserve. When turkey has completed roasting, remove it from the roasting pan, set roasting pan atop two burners, and over low heat add the 1/2 cup water to pan.
Blend water with pan drippings, using a wooden spoon to stir mixture and loosen browned bits from bottom of pan; the water and pan drippings together should equal about 1 cup. Reserve this liquid.
In a heavy saucepan, melt butter over medium-low heat. Whisk in flour; raise heat to medium and cook until flour mixture begins to turn golden brown about 2 minutes).
Stir in 1 cup of the reserved giblet stock and the reserved 1 cup of liquid from the roasting pan, stirring constantly until thickened. If mixture is too thick, whisk in additional turkey stock or, for a creamy gravy, add a little milk.
Season with salt and pepper. Stir in giblets and serve hot with roast turkey.
Servings: 2 cups