Clean, wash, and dice the carrot, celeriac, and onions.
Place the lamb in a roasting pan and arrange the diced vegetables around it.
Pour 200 ml of the lamb stock and the red wine over the lamb and vegetables.
Roast in the oven at 200°C (392°F) for 1 hour.
Reduce oven temperature to 180°C (356°F) and continue roasting for approximately 2 more hours, basting the lamb with pan juices every 20 minutes to keep it moist.
Remove the lamb from the roasting pan and set aside to rest.
Stir the remaining 200 ml of lamb stock into the roasting pan with the vegetables and pan juices.
Place the roasting pan on the stovetop over medium-high heat and bring to a boil.
Puree the mixture and strain through a sieve.
Bring the strained sauce to a boil over medium-high heat.
Sprinkle the sauce thickener into the sauce while stirring constantly until thickened.