Calculate cooking time — medium: 25 minutes per 500g plus 25 minutes; well done: 30 minutes per 500g, plus 30 minutes. Heat the oven to Gas 5, 190°C, fan170°C.
Using a sharp knife, make small deep slits all over the leg. Wrap a mint leaf around each piece of garlic and push into the slits in the lamb. Reserve the remaining mint.
Place the shallots, leeks, carrots and wine in a roasting tin and place the lamb on top. Drizzle with oil and then roast in the preheated oven for the calculated time, checking after 1 hour. If the vegetables are well cooked, remove and set aside so they don't burn (warm again in the oven just before serving).
Remove the lamb from the oven and lift it onto a serving plate before covering tightly with a sheet of foil. Leave the joint to rest for 15-20 minutes before carving.
Meantime, make the gravy. To make the gravy first pour away the excess fat from the roasting tin and then place tin with the remaining lamb juices over a medium heat on the hob. Sprinkle with the flour and then stir for a few seconds with a wooden spoon. Gradually add the stock and scrape up all of the caramelised juices from the base of the tin. Leave to simmer gently until it has reduced to a well-flavoured gravy, then strain into a gravy boat. Adjust seasoning, if necessary, and keep hot
If you can't find baby leeks, cut large ones in half crosswise and lengthways. Cold leftover lamb can be fried with peppers and onions for a tasty fajita.
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