The night before you plan to cook the lamb, put the oil, lemon juice, brown sugar, pepper, rosemary, tarragon, green onions, and garlic in a blender. Blend well to make the marinade.

Place the lamb in a large, heavy-duty, sealable food storage bag and add the marinade. Seal the bag, eliminating any air, and place it in a large baking or roasting pan. Put the pan in the refrigerator and marinate the lamb overnight and the next day, turning the bag over occasionally.

An hour or so before cooking, remove the pan from the refrigerator to let the lamb come to room temperature. Preheat a grill to high heat. Remove the lamb from the marinade. Transfer the marinade to a saucepan and bring to a full boil over medium-high heat.

Place the lamb on the hot grill and close the lid. Cook the lamb about 45 minutes, turning it over and basting it with the heated marinade every 10 minutes. The internal temperature of the lamb on an instant-read thermometer inserted into the thickest part should read about 130°F.

Transfer the lamb from the grill to a carving board, cover with heavy-duty aluminum foil, and leave to rest for about 15 minutes before carving. Garnish with additional sprigs of fresh rosemary.