Heat oven to 450ºF.
Place the turkey in a roasting pan. Using your fingers, separate the skin loosely from the meat. Distribute 15 sage leaves and the butter under the skin. Rub the skin with the oil and sprinkle with the salt. Roast 60 to 75 minutes or until an instant-read thermometer registers 170ºF.
Spoon 2 tablespoons of fat from the roasting pan into a small saucepan and add the chicken broth, mustard, and remaining sage leaves. Simmer for 5 minutes. In a small cup, whisk the flour and water until smooth, then stir it into the saucepan. Simmer, stirring constantly, until thickened, about 1 minute.
Carve one breast half into slices and serve with the gravy. You can slice the other breast half and freeze for later use. Pick off all the remaining meat from the carcass.