1 Combine marinade ingredients in a bowl. Lay the lamb in a large roasting tin then spread the marinade over the lamb. Massage the marinade into the meat, cover with clingfilm and transfer to the fridge for 2 hours or overnight.
2 Preheat the oven to 180°C, gas mark 4. Heat a large frying pan over a high heat, add the lamb and sear for a couple of minutes on each side until golden. Transfer to a clean roasting tin then roast in the oven for 40 minutes (for medium-rare). Remove from the oven and set aside to rest for 20 minutes. Turn the oven down to 150°C, gas mark 3.
3 For the plums, heat the sugar, cardamom and vinegar in a pan until the sugar has dissolved. Pack the plums into a small ovenproof dish then pour over the vinegar mixture. Transfer to the oven and roast for 15 minutes or until the plums are just soft. Remove from the oven and drizzle over the olive oil.
4 Combine the ingredients for the tahini cream in a small bowl and whisk until smooth.
5 Carve the lamb into slices and serve with the roasted plums and tahini lemon cream.