Spring Lamb Roast leg of lamb with rosemary and garlic ‘pesto’

1 x 2.25kg/5lb leg lamb
The leaves from 2 x 15cm/6 inch sprigs of rosemary, roughly chopped
3 garlic cloves
Finely grated zest 1/2 small lemon
4 tablespoons olive oil
Salt and freshly ground black pepper

Preheat the oven to 230C/450F/Gas Mark 8. To calculate the roasting time allow 16 minutes per 450g/1lb for medium (pink and juicy flesh nearest to the bone), 12 minutes for rare (very pink in the middle). Then, using a small, sharp knife, make little 2.5cm/1 inch deep slits all over the leg, about 5 cm/2 inch apart.
For the rosemary and garlic ‘pesto’, put the rosemary leaves, garlic, lemon zest, olive oil, 1/2 teaspoon salt and some freshly ground black pepper into a mini food processor and blend to a smooth paste. Push some of the ‘pesto’ into each slit, then rub the outside of the leg with the rest and season with salt and pepper.
Put the leg into a roasting tin and roast for 15 minutes. Then lower the oven temperature 200C/400F/Gas Mark 6 and continue to roast for 1 hour 15 minutes for a 2.25kg/5lb leg.
Remove the lamb from the oven, cover with a sheet of foil and leave to rest for 15 minutes before carving.