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Heat oven to 220C/200C fan/gas 7. Use a sharp knife to make little slits all over the lamb, then insert a slice of garlic into each hole. Tip the onions and carrot into a large flameproof roasting tin, then place the lamb on top and season. Pour 250ml water into the base of the tin and roast in the oven for 30 mins.
Meanwhile, make the crust by mixing together all the ingredients and seasoning well. Remove the lamb from the oven and turn the heat down to 190C/170C fan/gas 5. Brush the lamb all over with the mustard, then pack on the crust in a thick layer – don’t worry if any falls in the tin, this will be used in the gravy later (you probably won’t use the entire crust, so set aside what’s left for later). Return the lamb to the oven and cook for a further 1 hr 30 mins. Check the lamb during cooking and add more water to the base of the tin if it dries out.
Remove the lamb from the tin, cover lightly with foil and leave to rest for 30-40 mins – if covered too tightly, the crust will go soggy.
To make the gravy, put the roasting tin with the vegetables still in it directly on the heat and whisk in the flour. Cook for 1 min, then add the wine, scraping any flour from the bottom of the tin. Let it bubble for 1 min, then pour in the stock. Bring to the boil, then simmer gently for 10-15 mins until thickened, stirring in a little of the reserved crust for a thicker gravy. Strain the gravy through a sieve into a pan, discarding the vegetables left in the sieve. Add mustard to taste and stir through the butter. Keep warm until ready to serve.