1First of all, drain the chickpeas in a colander, rinse them under the cold tap then place them in the saucepan with 1.5l of boiling unsalted water. Then bring them up to simmering point, put a lid on and cook them very gently for about 1 hour or until the chickpeas are absolutely tender and squashy.
2While they're cooking, prepare the rest of the soup ingredients. The coriander and cumin seeds should be dry roasted in a small pre-heated pan for 2-3 minutes, then crushed in a pestle and mortar.
3After that, melt the butter in the pan, add the crushed spices along with the chopped garlic and chillies and cook over a low heat for about 5 minutes. Now add the turmeric, stir and heat that gently before removing the pan from the heat.
4As soon as the chickpeas are tender, drain them in a colander placed over a bowl to reserve the cooking water. Transfer the chickpeas to a liquidiser together with a couple of ladles of cooking water and purée them until fine and smooth. Now add the lemon zest, coriander stalks and spices from the pan along with another ladleful of cooking water and blend once more until fine and smooth.
5Next, the whole lot needs to go back into the saucepan with the rest of the reserved cooking water. Bring it all up to a gentle simmer, give it a good stir, season, then simmer gently for a further 30 minutes. All this can be done in advance, then, when you're ready to serve the soup, re-heat very gently without letting it come to the boil.
6Stir in half the soured cream and the lemon juice, taste to check the seasoning, then serve in hot soup bowls with the rest of soured cream swirled in.