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First, make the spice blend. Combine all the ingredients together in a spice grinder with 1 tbsp salt and blend to a fine paste. Alternatively, crush each ingredient with a pestle and mortar and combine to form a paste. Rub the paste onto the lamb fillets then leave in the fridge to marinate overnight.
Heat oven to 160C/140C fan/ gas 3. Heat 1 tbsp of the oil in a large flameproof casserole dish until smoking. Sear the lamb fillets in batches until evenly browned on all sides, then set aside on a plate. Good caramelisation on the meat will give it more flavour.
Wipe the pan clean with kitchen paper, then add the remaining oil. Once hot, add the shallots and cook over a medium-high heat, turning occasionally, until browned, about 5 mins. Once browned, add the flour and cook for 1 min to remove any floury taste.
Add the garlic and lamb stock to the casserole dish and bring to the boil, then return the lamb necks to the pan along with the merguez sausages. Cover with a tight-fitting lid and cook for 2 hrs. Can be braised a day ahead and left to cool.
Remove the hotpot from the oven and place overlapping slices of turnip and celeriac on top, pressing the first couple of layers into the sauce to absorb all of the delicious juices. Return the hotpot to the oven for 11/2 hrs until bubbling and golden. Serve at the table with mashed potato and pickled red cabbage.