Preheat the oven to 180 C / Gas 4. Season the lamb well with salt and pepper and brown off in a frying pan with a little olive oil. Remove and place on a baking tray.
Roast in the oven for 50 minutes to an hour. Depending on the size of your cut, keep an eye on it. You want a deep brown colour on the outside with crispy bits, but not too dry. You could always make ahead and keep warm or re-heat if unsure of cooking times.
Meanwhile prepare the rice and the sauce. Begin by placing the aubergines on a roasting tray with the pumpkin which has been seasoned. Place into the same oven as the lamb or in a separate oven. After 15 minutes, remove and turn the pumpkin over and turn the aubergine. It should be starting to colour as should the aubergine. After another 10 to 15 minutes remove the pan and check the pumpkin. It should be roasted and nicely coloured. If necessary place back in the oven until done. Remove the pumpkin and set aside. Continue to cook the aubergine until blackening and tender.
When the aubergine is cooked, remove and place in a colander as there will be plently of juices ready to escape. Allow to rest until cool enough to handle. Remember to keep checking the lamb if not cooking ahead.
Now for the messy bit - remove the skin from the aubergine and any membrane inside. Chop into chunks and and place into a blender or puree with a hand blender. Season and allow to cool.
To make the rice, heat some oil in a saucepan and add the onion and after a few minutes the garlic. Stir well. Add the rice and stir to coat each grain. Now add the water and stock and season with salt and pepper. Cover and cook for about 12 minutes. Stir occasionally. Check the tenderness of the rice and cook longer if needed. Remove the lid if too much liquid remains towards the end of cooking to evaporate. You may need to vary the amount of liquid used. You should end up with fluffy cooked rice. Keep warm while you prepare the rest.
Add the yoghurt to the aubergine puree and mix well. Pour into a plastic squeezy sauce bottle and set aside. To the rice stir in the pumpkin, sultanas and pine nuts. Mix well.
Remove the lamb from the oven and allow to rest. Then slice into chunks.
To assemble, place a deep cooking ring on each serving plate towards the side. Fill each ring with the rice mixture then arrange the lamb chunks around this. Drizzle the plate with the aubergine puree decoratively and place a little of the puree on top of the rice. Garnish the rice with mint and serve.
The aubergine puree can be made a day ahead and chilled.