1. Use a paring knife to make a dozen small slits in the meat. Tuck garlic slivers into slits. Place meat in a baking dish.

2. In a small bowl, stir together oil, lemon juice, oregano, thyme, rosemary, 1 teaspoon salt, and 1 teaspoon pepper. Pour marinade over lamb, turning to coat. Cover with plastic wrap, and refrigerate, basting occasionally with the marinade, for at least 8 hours and up to 24 hours.

3. Preheat oven to 325 degrees with rack positioned to fit the lamb in a roasting pan. Remove lamb from refrigerator, uncover, and let sit at room temperature for at least 30 minutes. Using a pastry brush, lightly coat a roasting pan with oil.

4. Transfer lamb to a rack fitted in the prepared pan, reserving leftover marinade. Season meat with salt and pepper. Roast lamb, basting every 20 to 30 minutes, first with the reserved marinade, and then with juices that accumulate in the pan. The lamb will need to cook for about 20 minutes per pound (2 hours for a 6-pound leg of lamb; 2 hours and 40 minutes for 8 pounds). An instant-read thermometer inserted in the thickest part of the meat, not touching the bone, should read 145 degrees when the lamb is medium-rare.

5. Remove pan from oven and transfer lamb to a cutting board. Cover to keep warm; let rest for 20 minutes before carving. Pour pan juices into a fat separator or glass measuring cup; spoon off any fat that rises to the surface and discard. Cover juices to keep warm while lamb rests; add any juices that collect as the lamb sits.

6. Carve lamb, and arrange slices on a large warm platter; drizzle with reserved pan juices just before serving.