Using a food processor, whizz the coriander seeds with the garlic, fresh coriander, mint and half the chickpeas until you have a paste. Season the paste with salt and pepper, then add lemon juice and yoghurt. Place half the flavoured yoghurt in a large plastic bag and add the lamb. Put the other half of the yoghurt mixture in the fridge. Tie the bag and seal it. Leave to marinade overnight in the fridge.
Preheat the oven to 200ºC/400ºF/Gas 6. Place the vegetables in a roasting tray, then sprinkle with cumin, olive oil, salt and pepper. Toss together to coat.
Remove the lamb from the marinade, then place the meat directly on the oven rack with the tray of vegetables on the shelf below. Cook for about 1 hour, tossing the vegetables halfway. Serve the well cooked lamb with the vegetables and the flavoured yoghurt on the side.