Rinse lamb and pat dry; trim off excess surface fat. With a small, sharp knife, cut slits about 1/2 inch wide and deep all over lamb and insert slivers of garlic.
In a 9- by 13-inch noncorrosive baking dish or large bowl, mix wine, olive oil, lemon juice, sherry, thyme, rosemary sprigs, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Add lamb and turn to completely coat with marinade. Cover the lamb and chill, turning occasionally, at least 2 hours or up to overnight.
Lift lamb from marinade and spread out flat on an oiled barbecue grill over a solid bed of medium coals or medium heat on a gas grill (you can hold your hand just above grill level only 4 to 5 seconds). If cooking over charcoal, drop rosemary sprigs from marinade onto coals for aromatic smoke, if desired. Discard marinade. Close barbecue lid. Cook lamb, turning once, until a thermometer inserted in center of thickest part registers 135° for medium-rare (still slightly pink; cut to test), 20 to 35 minutes; thin parts will be well done.
Transfer lamb to a rimmed cutting board or platter, cover lightly with foil to keep warm, and let stand 10 to 15 minutes. Slice meat thinly and sprinkle with salt and pepper to taste.
Note: Nutritional analysis is per serving.