Preheat the oven to 220°C / 425°F / Gas mark 7.
Use a sharp knife to make little slits all over the lamb, then insert slices of garlic into each.
Tip the onions and carrot into a large roasting tin, then place the lamb on top and season with salt and pepper.
Pour 250ml of water into the base of the tin and roast for 30 minutes.
Mix together all of the crust ingredients and season generously with salt and pepper.
Remove the lamb from the oven and turn the heat down to 190°C / 375°F / Gas mark 5.
Coat the lamb all over with the mustard, then thickly layer it over the lamb.
If some falls into the tin that is ok and will this will be used in the gravy later.
Return the lamb to the oven and cook for one hour and 30 minutes.
Check the lamb during the cooking time and add more water to the base of the tin if necessary.
Remove the lamb from the tin, cover lightly with foil and leave to rest for 30-40 minutes. Do not cover tightly.
Put the roasting tin with the vegetables still inside directly on the hob heat and whisk in the flour.
Cook for 1 minute, then add the wine, scraping any flour from the bottom of the tin.
Let it bubble up for 1 minute or so, then pour in the stock.
Bring to the boil and simmer for 10-15 minutes until thickened.
Stir in a little of the reserved crust for a thicker gravy if you like at this point.
Strain the gravy through a sieve into a pan, discarding the vegetables.
Add the mustard and stir through the butter and serve or keep everything warm until ready to serve.