Cut 8 small slits over the surface of the lamb. Tuck a half garlic clove in each slit.
Using kitchen string, tightly tie the lamb in several places (see photo above).
In a small bowl, stir together the olive oil, rosemary, salt and pepper. Rub the rosemary mixture over the surface of the lamb.
Place the leg of lamb in a roasting pan or on a rimmed baking sheet. Cook at 425 degrees F for 15 minutes.
Decrease the heat to 350 degrees F and roast until a thermometer inserted into the thickest part of the meat reads 130 degrees F, about 1 ¼ hours. In the last 5 minutes of cooking, brush the lamb with ⅓ of the glaze.
Transfer to a cutting board and let rest for about 20 minutes. The temperature will rise about 10 degrees as the lamb rests, bringing it to medium-rare.
Brush the lamb with another ⅓ of the glaze. Slice against the grain. Serve with remaining glaze.
In a small, whisk together the blackberry preserves, Worcestershire sauce and apple cider vinegar.