How to Make It
Preheat the oven to 400°. In a small baking dish, toss the garlic with 1 tablespoon of the olive oil. Cover with foil and bake for 1 hour, until the garlic is golden. Let cool slightly, then squeeze the cloves out of the skins. Transfer the garlic to a mini food processor. Add the salt, pepper, oregano, thyme and the remaining 2 tablespoons of olive oil; process to a paste.
Arrange the leeks in the bottom of a large roasting pan, cut sides up, and drizzle with olive oil and the wine. Rub 1 tablespoon of the garlic paste on the underside of the lamb and lay it on the leeks. Spread the remaining garlic paste over the lamb and roast for 30 minutes. Lower the oven temperature to 350° and roast the lamb for about 1 hour and 10 minutes, until an instant-read thermometer inserted into the thickest part of the meat registers 130° to 135° for medium-rare. Transfer to a carving board and let rest for 20 minutes.
Turn the leeks and roast them for about 5 minutes longer, until the liquid in the roasting pan is nearly evaporated. Carve the lamb, transfer to a platter and serve with the leeks.