Preheat the oven to 220C.
Peel the potatoes, cut them up into large pieces and tip them into a large saucepan. Cover with plenty of salted water and set them over the heat. Bring to the boil, turn down the heat and simmer for 8 - 10 minutes or until they are just tender. Don’t let them start to break up though.
Drain the potatoes, place them into a colander and leave to cool, for at least 5 minutes. It’s well worth tumbling the potatoes around in the colander, once or twice, to rough up their edges, as this makes them even crispier.
Meanwhile spoon the beef dripping into a large heavy based roasting tin and place it in the oven to heat up. Once it’s lovely and hot, carefully turn the potatoes out into the tray. Use a spatula to turn them through the beef fat, making sure they’ve all got a little space.
Sprinkle the potatoes all over with sea salt and season them with freshly ground black pepper.
Season the leg of Mutton with salt and add to the roasting tin along with the halved garlic bulbs, nestling the potatoes in and around the joint.
Place the roasting tin in the middle of the hot oven and roast for 20 minutes.
After 20 minutes, remove the roasting tin from the oven. Carefully press the potatoes to squish them lightly with a potato masher. Add the rosemary and thyme.
Turn the oven down to 180C and cook for a further 25 minutes per kilo of the wight of the mutton leg. For best results use a meat thermometer. Place the tip in the thickest part of the joint (not next to the bone) the target temperature is 55C.
Remove from the oven and leave to rest for 20 minutes. Take the roasted garlic from the roasting tin and set to one side as you’ll need this to make the pesto.
While the mutton is resting, make the mint pesto.
Lightly toast the walnuts in a frying pan. Once toasted, add them to a blender, followed by the mint, chives, parsley, roasted garlic, olive oil, a pinch of salt and pepper and a dash of vinegar. Blend until you have reached your desired consistency.