Remove fell from lamb (parchment-like covering); trim excess fat.
Place on rack in shallow baking dish, fat side up.
Roast at 325°F. until done (30 to 35 minutes per pound or 175°F. on meat thermometer).
Meanwhile, in saucepan, cook onion with garlic in butter until tender; add remaining ingredients.
Cook over low heat, stirring until thickened.
Brush lamb with sauce during last 30 minutes.
Heat remaining sauce.