1. Trim off and discard any surface fat on meat. Lay lamb boned side up. With a small sharp knife, make small slashes (1 inch long, 1/2 inch deep) all over meat surface. Cut the rosemary sprigs into 1-inch pieces. Tuck a rosemary piece and garlic slice evenly into each slash.
2. In a deep noncorrodible bowl or 2-gallon heavy-plastic food bag, mix wine, sugar, soy sauce, and mustard. Add lamb, and turn to coat evenly, being careful that garlic and rosemary remain in slashes. Cover or seal, and chill, turning occasionally, at least 2 hours or up to 1 day.
3. To prepare barbecue, ignite 60 charcoal briquets in a barbecue with a lid. When the coals are mostly covered with gray ash (about 30 minutes), push equal portions to opposite sides of the firegrate; place a drip pan in the center. To maintain temperature, add 6 briquets to each side. (Or turn gas barbecue on high, and heat, covered, for 10 minutes, then adjust for indirect cooking.) Place a lightly oiled grill 5 to 6 inches above the coals.
4. Lay lamb on grill; reserve marinade. Brush meat with half the reserved marinade.
5. Cover barbecue (open vents for charcoal). Cook until a thermometer inserted into the thickest part of meat registers 140°, about 40 minutes. Brush halfway through with remaining marinade.
6. Transfer meat to a carving board. Garnish with additional rosemary sprigs. Cut into thin slices to serve.