6 garlic cloves
9 flat anchovy filets, drained and patted dry
1/4 cup olive oil
2 1/2 tablespoons chopped fresh rosemary
1 (6 to 7 pound) bone-in leg of lamb
2 teaspoons salt
3/4 teaspoon black pepper
Remove all but a thin layer of fat if leg is fatty. Mince garlic and anchovies and mash to a paste with a large heavy knife, then stir together with oil and rosemary in a small bowl. Pat lamb dry and transfer, fat side up, to rack in pan. Make several small 1-inch-deep slits in lamb with a paring knife, then rub marinade over entire surface of lamb, pushing some marinade into slits. Marinate lamb, loosely covered, at room temperature 1 hour.
Put oven rack in middle position and preheat oven to 400ºF.
Sprinkle lamb all over with salt and pepper, then roast until thermometer inserted into thickest part of lamb (almost to the bone but not touching it) registers 125ºF for medium-rare, 1 1/2 to 1 3/4 hours (temperatures in thinner parts of leg may register up to 160ºF). Note that the lamb cooks faster than you think. If you prefer rare meat, take care not to overcook.
Let stand 15 minutes before slicing. Lamb will continue to cook while standing.
NOTE: If roasting a half leg of lamb, shorten your cooking time by at least 1/2 hour. An instant read meat thermometer is your friend. Check meat temperatures often to avoid overcooking.