Preheat oven to 150.
Season the lamb shoulder with olive oil, salt and pepper and place in a large baking dish. Add the lamb stock, 2tbsp of cider vinegar and the sugar. Tightly tin foil the dish and slowly cook for 5 hours.
After 5 hours, check the lamb is tender, it should be very soft to the touch, if not return to the oven for another hour.
Remove the lamb from the dish and strain the juices into a saucepan and skim off as much of the fat as possible. Set the pan on the heat to reduce the liquid into a glossy glaze.
Cut the beetroots into matchsticks and finely slice the red cabbage and place into a large bowl. Dress with olive oil, sea salt, black pepper and a couple of tbsp of cider vinegar, massaging the dressing into the veg until it softens slightly.
Season the yoghurt with salt and pepper.
Pour the reduced glaze over the lamb shoulder and return the oven basting every 5 minutes or so until the lamb is sticky and nicely glazed.
Place the vegetables onto a platter and scatter with the chervil and dot over the seasoned yoghurt. Serve alongside the lamb with a big pile of flatbreads for everyone to share.