Put the chickpeas in a bowl, cover with three times their volume of water, add the bicarb and soak overnight. Drain, put in a pan, cover with water and simmer until soft.
Dry-roast the coriander, mustard and cumin seeds in a skillet, then crush to a powder. Mix in the curry powder, turmeric, sugar and salt.
In the same pan, heat half the oil and, on high heat, cook the onion for five minutes, stirring occasionally. Add the spice mix, cook on medium heat for five minutes, until the onion is soft, then transfer to a large bowl.
Blanch the cauliflower for a minute, then drain. Once it is dry, heat the remaining oil in the same pan you cooked the onion in, add the cauliflower and fry on high heat for two minutes, to give it colour. Add the cauliflower and hot chickpeas to the onions (the chickpeas absorb more flavour when hot), stir and leave to cool to room temperature.
Peel the mangoes, cut into 1cm dice and add to the bowl. Stir in the chilli, fresh coriander, lime juice and spinach, taste, and add salt and lime juice as needed. Serve at once, or chill and serve within 24 hours.