Foolproof Make-Ahead Gravy

Bring all ingredients except butter and flour to boil in a large saucepan. Reduce heat and simmer, uncovered, 11 ⁄2 hours Remove wings to a cutting board to cool. Pull off skin and meat. Discard skin, dice meat and add to gravy, or save for another use Strain broth into a large pot, pressing vegetables to extract as much liquid as possible. Discard vegetables. Skim fat off surface of broth and discard Mash butter and flour with a fork until blended to a paste. Divide in 4 portions Bring broth to a boil, reduce heat to low and gradually whisk in flour mixture, 1 portion at a time, until blended. Increase heat and whisk until thickened and boiling. Boil 3 minutes to cook out "floury" taste Cover surface of gravy with waxed paper or plastic wrap (to keep a skin from forming) and refrigerate