If you can, prepare your lamb the day before you want to cook it. Using a sharp knife, carefully cut down the lamb leg to expose the bone.
Keeping your knife as close to the bone as possible, cut along one edge to completely reveal it.
Cut right around the bone, then remove it.
Lay the lamb flat on a chopping board, skin-side down. With your knife, make an incision halfway into the flesh on each side, where the meat is thicker, then open it out like a book.
You should now have a flat piece of lamb of more or less even thickness.
Soak your dried chillies in a bowl of hot water for 10 minutes.
Meanwhile, in a small frying pan over a medium heat, dry-toast the black and pink peppercorns, fennel and coriander seeds for 30 seconds, until they begin to pop and smell delicious.
Remove the softened chillies from the soaking water and transfer to a spice grinder or small food processor. Add the toasted spices and a splash of the soaking water, then blitz to a fine paste.
In a large bowl, mix the spice paste with the lemon zest, paprika, oil, vinegar and a pinch of sea salt. Pick in the oregano leaves, peel and crush in the garlic, then finely slice and stir through the chilli until combined. Add the butterflied lamb to the bowl and rub the marinade all over the meat. Cover and refrigerate for at least 4 to 6 hours, but ideally overnight.
Remove from the fridge at least 30 minutes before you're ready to cook, so the lamb comes up to room temperature. Fire up your barbecue, then once hot, cook the meat for 40 to 45 minutes, or until medium-rare, turning it every 10 minutes or so.
Transfer the lamb to a board and allow to rest for 15 minutes while you make your salad.
Click off and combine the chicory leaves with the watercress and pomegranate seeds in a serving bowl.
In a small bowl, stir the harissa through the yoghurt and season to taste.
Slice the lamb on the board and halve the zested lemon. Serve the lamb on the board with the salad, yoghurt dressing and lemon halves on the side.
Ancho chillies are dried poblano chillies, used in Mexican cuisine. If you can’t find them at your supermarket, you can get them at souschef.co.uk.