Rinse prunes, cut into halves, and remove and discard pits.
Rub fat side of meat with a mixture of the salt, pepper, and ginger.
Lightly mark the fat side at about 1/2 inch intervals to indicate slices.
Cut 2 or 3 pockets along each line and insert prunes so they are completely embedded in meat.
(Some prunes come to the top of the meat by the end of the roasting period.) Insert meat thermometer so that tip is slightly beyond center of thickest part of meat, not in fat or on bone.
Place roast, fat side up, on a rack in a shallow roasting pan.
Roast, uncovered, at 325° to 350°F until internal temperature registers 170°F, about 1 1/2 to 2 hours.
(Allow 30 to 35 minutes per pound.) Transfer roast to heated serving platter and keep warm while preparing gravy.
Pour off all but 1/4 cup of drippings (to be used for gravy).
Drizzle some of remaining drippings over roast.