Unwrap ham, and remove plastic disk covering bone. Tuck rosemary sprigs randomly between every 2 or 3 slices of ham. Place ham, cut side down, in an oven cooking bag; place in a 13" x 9" baking dish. Close bag with tie. Trim excess plastic to 2". Cut 3 (1/2") slits in top of bag. Bake at 275° on lowest rack for 2 1/2 hours or until a meat thermometer inserted registers 140°.
Remove ham from oven; let stand in bag with juices 15 minutes. Remove ham from bag, and transfer to a carving board. Separate slices, and arrange on a serving platter. Garnish, if desired. Serve with Cumberland Sauce.