Trim excess fat from leg of lamb.
Using a sharp knife, cut several small slits on outside of lamb; stuff with garlic slices.
Combine flour, salt, ginger, mustard, and pepper, stirring well; rub over surface of lamb.
Set lamb aside.
Combine chili sauce, oil, worcestershire sauce, and vinegar, stirring until blended.
Set sauce aside.
Place lamb, fat side up, in a shallow roasting pan.
Insert meat thermometer into lamb, making sure it does not touch bone or fat.
Arrange onion slices around lamb.
Baste with sauce.
Bake, uncovered, at 400° for 25 minutes.
Reduce heat to 350°, and bake an additional hour and 15 minutes or until thermometer registers 140° (rare) or 160° (medium).
Baste lamb every 15 minutes with sauce.
Add boiling water to pan during last hour of baking time.
Transfer lamb and onion slices to a serving platter, discarding pan drippings.
Let stand 10 minutes before slicing