Take the lamb out of the fridge at least 1 hr before cooking to come to room temperature. Score the skin into 3cm diamonds using a sharp knife, but don’t cut too deep.
Cut the garlic bulb in half to expose the flesh of each clove. Set one half aside, then pop the cloves from the remaining half out of their skins. Put these in a small food processor along with most of the mint and parsley, the rosemary, lemon zest, preserved lemon and a large pinch of salt. Blitz the mixture together, or pound using a pestle and mortar, adding half the olive oil once the herbs have started to break down. Pour the marinade over the lamb, massaging it into the cuts. Leave to marinate at room temperature for at least 30 mins, or chill for up to 24 hrs.
Heat the oven to 200C/180C fan/gas 6. Put the lamb in your largest roasting tin (you’ll need space to add the potatoes later). Wrap well with foil and bake for 45 mins.
Meanwhile, put the potatoes in a large pan of cold salted water. Set over a medium heat and bring to the boil, then reduce the heat and simmer for 5 mins. Drain and leave to steam-dry for 10 mins.
Remove the lamb from the oven and lift onto a board. Drizzle the remaining oil into the roasting tin, then tip in the potatoes, fresh lemon slices and the remaining garlic bulb half, rubbing the cut side in the oil. Season and toss to coat the potatoes and lemon slices in the oil and lamb juices. Sit the lamb on top.
Return to the oven for 35 mins more for medium (a thermometer should read 55C), or 50 mins for well done. Transfer the lamb to a warm platter, cover and leave to rest. Meanwhile, shake the potatoes around in the tin and return to the oven for another 20-25 mins, or until golden and crisp. Swirl the harissa paste through the yogurt.
Roughly chop the remaining mint and parsley. Scatter the chopped herbs and feta over the potatoes. Carve the lamb and pour any roasting juices into the spiced yogurt, and mix through. Serve the lamb and potatoes with peas or spring greens, and the spiced yogurt sauce on the side for drizzling over.