Ideally, marinate the lamb the night before: mix the spices, orange zest, pomegranate molasses and olive oil together in a bowl and season with black pepper only. Using a sharp knife, lightly slash the lamb 4 or 5 times across the top. Coat the lamb in the marinade and put on a plate. Cover and chill for at least 1 hour or up to 48 hours.
Preheat the barbecue. Whisk all the dressing ingredients together in a bowl – add a little cold water to thin it down; season and set aside.
When the barbecue is ready, season the lamb with salt, then cook for 25-30 minutes, turning halfway (for medium rare). If not using a barbecue, cook on a preheated griddle pan, over a medium high heat for 5 minutes each side, then transfer to a preheated oven (240°C, fan 220°C, gas 8) for 20 minutes. Transfer to a board and leave to rest for 10 minutes.
Meanwhile, for the salad, slice the radishes, halve the tomatoes and chop the spring onions and cucumber. Add to a large bowl with the lettuce. Add half the pomegranate seeds, pistachios and the shredded mint to the bowl. Season and toss everything together.
Scatter the remaining pomegranate seeds, pistachios and mint leaves over the lamb, and drizzle with a little tahini dressing and pomegranate molasses. Drizzle the rest of the tahini dressing over the salad. Carve the lamb and serve with the warm flatbreads.