Remove the giblets, then rinse the turkey. Dry with paper towels. Rub the cavity with the salt. Cut 2 of the oranges into 2-inch chunks. Set aside a few sage sprigs for garnish later. Place the remaining sage, cut-up oranges, and garlic in the cavity. Loosen the skin from the breast and spread the cheese under the skin. Tuck the wings under the back; tie the legs together. Arrange the carrots and leeks in the roasting pan to create a "rack" and place the turkey on it; rub with oil. (The turkey can be prepared to this point up to 1 day ahead. Cover the pan with plastic wrap and refrigerate.) Heat oven to 350ºF. Roast for 1 hour. Place a foil tent over the turkey and continue roasting for 2 1/2 to 3 hours more or until an instant-read thermometer registers 180ºF when inserted in the breast.
Remove the roasting pan from the oven. Carefully move the turkey to a platter, pouring any cavity juice back into the pan. Cover with foil to keep warm. Set oven to 500ºF. Cut the remaining 6 oranges into quarters and arrange in a shallow baking pan. Roast for 10 to 15 minutes or until brown.
Meanwhile, discard the vegetables from the roasting pan. Pour the pan drippings into a 4-cup measure. When the fat separates and rises to the surface, spoon 1/3 cup of it into a medium saucepan; discard any remaining fat. Squeeze the juice from 12 of the orange wedges into the defatted broth; if necessary, add water or chicken broth to make 4 cups. Whisk the flour into the fat in the saucepan and cook over medium heat, stirring constantly, for 3 minutes. Add the broth mixture and cook, stirring constantly, until the gravy thickens and boils, 5 to 7 minutes.
Arrange the remaining baked oranges with the sage on the platter around the turkey. Serve with the gravy.
Time-savers: Use a bed of carrots and leeks instead of a roasting rack. The vegetables flavor the drippings, and there's no rack to scour. Spread garlic-herb cheese under the turkey skin instead of the traditional mixture of butter and herbs. It adds flavor to the turkey and eliminates extra chopping and dirty mixing bowls. Roasting the turkey unstuffed shaves about 30 minutes off the cooking time.