A week before you intend to cook, add chopped dried cherries to a clean pint jar with 1 cup of port wine and shake. Set in a cool dark place until roasting day.
While the duck roasts, transfer port and fruit mixture to a deep, heavy saucepan. Bring to a rapid simmer over moderate heat. Light fumes with a long lighter and flambé. Note: it is dangerous to flambé in a kitchen. This should only be performed by an experienced adult on a well-ventilated, clear stovetop.
Once the flames have died down, the mixture will be thickened. Stir in the frozen cherries and heat until the mixture is warmed through. Remove from heat.
Once the duck is finished and resting in another container, carefully pour off the duck fat into a clean, heat-resistant container and save for later use. Place the roasting pan on a stove burner (balance it across two if required) over moderate heat. Deglaze with the remaining port and transfer to the saucepan. Stir the mixture and reheat at medium low until hot and thick. Serve alongside duck in a bowl or gravy boat.