Preheat oven to 425º F.
Cut slits all over the lamb with the tip of a sharp knife and insert the garlic slivers in the slits.
Brush the lamb all over with 4 tablespoons of olive oil.
Combine the 2 tablespoons thyme, the dried rosemary, 2 tablespoons pepper, and coriander in a small bowl and mix well. Pat the herb mixture all over the lamb to form a crust. Place the lamb in a shallow roasting pan.
Place the potatoes in a large mixing bowl. Add the remaining 5 tablespoons olive oil, the fresh rosemary, the remaining 2 teaspoons thyme and 1 teaspoon pepper and the coarse salt. Toss to mix well, and arrange around the lamb in the pan.
Place the pan on the center rack in the oven, and roast for 45 minutes. Then reduce the heat to 375 degrees F, stir the potatoes slightly so they don’t stick to the pan and cook an additional 30 minutes.
Remove the pan from the oven, and insert an instant-reading meat thermometer in the thickest part of the meat. It should read 120 degrees F for rare. Let the lamb rest, loosely covered, 15 minutes before carving. (The meat will continue to cook a bit more, and the temperature will rise to 135 degrees to 140 degrees F.) Place the roast on a serving platter, surrounded with the potatoes. Garnish with fresh rosemary sprigs.
For rare lamb allow 12 minutes per pound. For well-done lamb, allow 18 minutes per pound.
* Preparing the leg of lamb: Roasting a leg of lamb with the bone will impart a stronger flavor and will retain the heat well. For easier carving, you can have it boned and then have the bone put back inside, tying the roast with string. Otherwise, you can leave the bone as is and carve around it.
Servings 8