How to Make It
Preheat the oven to 400°. Rub 1/2 tablespoon of the olive oil all over the lamb and season with salt and pepper. In a medium roasting pan, heat the remaining 2 tablespoons of olive oil. Add the onions, carrots, celery, leek, thyme, parsley sprigs, head of garlic and bay leaf. Cook over moderate heat, stirring occasionally, until the vegetables have softened, about 10 minutes. Add the water and wine and bring to a boil.
Set the lamb on the vegetables, fat side up, and put the roasting pan in the oven. Reduce the oven temperature to 325° and roast the lamb for about 3 hours, turning it twice to brown the exposed sides. The lamb should be very tender.
Transfer the lamb to a carving board, cover loosely with foil and let rest for 10 minutes. Strain the pan juices into a medium saucepan, pressing on the solids to extract as much liquid as possible. Skim the fat from the surface and boil the juices over high heat until reduced by half, about 8 minutes. Season with salt and pepper.
Finely chop the lemon zest with the chopped parsley and the garlic clove, then transfer this gremolata to a small bowl. Thickly slice the lamb and transfer it to a platter. Sprinkle the gremolata over the lamb and serve with the pan juices.