Remove the fell (tissue-like covering) from lamb with a sharp knife. Make small slits on outside of lamb; set aside.
Combine flour, ginger, mustard, salt, and pepper; mix well. Rub surface of lamb with flour mixture, coating well. Set aside.
Combine chili sauce, olive oil, Worcestershire sauce, and vinegar, stirring until well blended; set aside.
Place lamb, fat side up, in a shallow roasting pan. Insert meat thermometer, if desired. Arrange onion slices and garlic around lamb. Baste with chili sauce mixture.
Bake, uncovered, at 400° for 25 minutes. Reduce heat to 350°, and continue baking until desired degree of doneness: about 1 hour and 15 minutes or 140° (rare); about 1 hour and 30 minutes or 160° (medium); about 2 hours or 170° (well done). Baste every 15 minutes with chili sauce mixture. During last hour of baking add boiling water to pan.
Transfer lamb to a warm serving platter, reserving pan drippings. Let lamb stand 10 minutes before slicing.
Skim fat from pan drippings; discard fat. Spoon pan drippings over lamb to serve.