6 large potatoes (Maris Piper or similar), thinly sliced
salt and freshly ground black pepper
Preheat the oven to 170C/150C Fan/Gas 3½.
To make the lamb, heat the oil in a large ovenproof casserole dish over a medium heat. Brown the lamb shanks on all sides, then transfer them to a plate or board. Add the onion, garlic, celery, peppercorns and herbs to the pan and cook for 5 minutes.
Add the lamb back to the pan. Pour in the wine and simmer until reduced by half. Add the stock, cover with the lid and bake for 2½ hours, or until tender.
Meanwhile, to make the dauphinoise, place the cream, thyme and garlic in a saucepan and season with salt and pepper. Bring to the boil and remove from the heat immediately.
Lay some of the potatoes in a single layer in an ovenproof dish and cover with some of the cream mixture. Repeat this in layers until all the potatoes and cream have been used. Bake for 1–1¼ hours, or until golden and a knife easily goes all the way through.
When the lamb is cooked, strain the liquid into a saucepan. Put the lamb shanks into a roasting tin and roast for 10–15 minutes to brown. Simmer the liquid over a medium heat until reduced to the consistency of double cream. Whisk in the butter and keep warm.
Serve the lamb with the dauphinoise potatoes and pour over the sauce.