Preheat the oven to 200C/400F/Gas 6.
Using a sharp knife, prick the leg of lamb all over, about 20 times and about 2cm/1in deep.
Cut the garlic cloves into thin strips and place one slice of garlic into each hole with a few rosemary leaves. Continue until all the holes are full. Season with salt and freshly ground black pepper and drizzle with oil.
Place the lamb directly onto the oven rack and place a roasting tray directly underneath on the rack below. This is to catch the fat as it drips from the lamb. Roast in the pre-heated oven for 1 hour 15 minutes or until cooked to taste, basting frequently.
Par-boil the potatoes. Drain them, then return them to the pan and shake to roughen the edges.
After the first 15-20 minutes of cooking the lamb, carefully tip the potatoes into the roasting tray underneath the lamb, which will now contain the hot fat from the lamb above.
Coat the potatoes in the fat then roast with the lamb until golden and cooked through, turning once or twice.
To make the pesto, put all the ingredients for the coriander pesto, except the oil and seasoning, into a food processor. Blend, gradually adding oil until you reach the desired consistency. Season with salt and freshly ground black pepper. Serve with the lamb and roast potatoes.