Prick the aubergines all over with a fork. If you have a gas hob, lay the aubergines onto the flame for 15-20 mins, turning them every 5 mins. They should be deflated and the skin completely blackened. Alternatively, heat your grill to its highest setting. Place the pricked aubergines under the grill for up to an hour, turning them every 10-15 mins till the skin is blackened.
While the aubergines are cooking, pat the diced lamb dry with kitchen paper. Set a large heavybased pan on a high heat for a couple of mins. When warm, drizzle in 1 tbsp oil. Add ½ of the lamb and fry for 4-5 mins, stirring now and then till the lamb is browned. Transfer with a slotted spoon to a bowl. Add ½ tbsp more oil if you need to and repeat with the remaining lamb. Cook till browned then add to the bowl with the rest of the lamb.
While the lamb is frying, peel and slice the onion. Halve the pepper and chop into 1-2cm sized pieces. Peel and thinly slice the garlic.
When the lamb is cooked, keep the pan on the heat. Add ½ tbsp olive oil. Add the onions and peppers. Fry for 4 mins till the veg is softened and slightly caramelised at the edges. Add the sliced garlic and stir together.
Add the chopped tomatoes, bay leaf, cinnamon stick and a pinch of cayenne. Crumble in the stock cube and 300ml boiling water. Bring to a bubble then add the lamb back to the pan along with any resting juices. Cook for 40 mins till the lamb is tender and the sauce is thick. Stir now and then, adding a splash of water if the sauce looks too dry.
Halve the aubergines lengthways as soon as they are cool enough to handle. Scoop out the flesh with a spoon. Don’t worry if there are still little bits of the blackened skin. Roughly chop. Place in a colander to drain.
When the lamb has 10 mins left to cook, melt the butter in a saucepan then add the flour. Stir till you have a thick paste. Slowly pour in the milk, whisking all the time till smooth. Cook for 4 mins till thickened. Remove from the heat. Grate in ½ the nutmeg. Add a large pinch of salt and a good crack of pepper. Add the aubergine and mix well.
Taste and adjust the seasoning of the lamb and aubergine purée. Spoon the aubergine purée into a couple of warm shallow bowls. Top with the lamb.