5 cups canned low-sodium chicken broth, or more if necessary
2 lbs. fresh mushrooms, thickly sliced
1/3-cup instant flour
2 green onions, chopped
Strain juices from roasting pan into bowl, pressing hard on solids and scraping up any browned bits. Spoon fat from top of juices, reserving two tablespoons of fat, or use a gravy separator to separate fat from juices. Strain juices into a large, heat-proof measuring cup. Add remaining half cup wine and enough chicken broth to turkey pan juices to measure five cups. Heat reserved two tablespoons turkey fat or two tablespoons butter in a heavy, large pot over medium-high heat.
Saute mushrooms until they're brown and the juices evaporate, about 18 minutes. Gradually sprinkle in flour, stirring continuously to incorporate, until flour begins to brown, about two minutes. Gradually whisk in the five cups of broth mixture. Bring to a boil, scraping up browned bits, and continue to boil until gravy coats a spoon lightly, about 8 minutes. Salt and pepper to taste. Pour in a gravy boat and serve with turkey.
A gravy separator is available in housewares and kitchen supply stores.