Place the lamb in a large freezer bag. Coat it with the buttermilk, and fasten the bag securely. Leave in the fridge to marinate overnight or preferably for 24-48 hours.
When you are ready to cook, preheat the oven to 160C/325F/Gas 3. Place the lamb with the buttermilk and 200ml/7fl oz water in a deep casserole or ovenproof pan. Season with plenty of salt and pepper. Cook the lamb in the oven for 3½ hours, basting it every hour or so with the cooking liquid. When done, remove from the oven and cover with lightly greased aluminium foil to rest for 10 minutes.
For the salad, tip the buckwheat into a large saucepan and dry-toast for about five minutes, or until it is golden-brown and has a nutty fragrance. Remove from the heat and leave to cool.
Tear the mint and parsley leaves into the buckwheat, add a pinch of salt, the lemon juice and oil. Toss together then set aside.
To make the yoghurt sauce, mix the lemon zest and juice with the yoghurt, onion and sugar. Season to taste with salt and pepper.
To serve, flake the meat and serve on top of the buckwheat salad with the yoghurt sauce drizzled over.