Soak the chickpeas overnight in 2 litres (8 cups) water. Drain, then put the chickpeas in a large saucepan with 2 litres (8 cups) water. Bring to the boil, spooning off any scum from the surface. Cover and simmer over low heat for 1-1½ hours, until soft. It is important they are soft at this stage as they won’t soften once the sauce is added. Drain.
Heat the oil in a karhai or heavy-based frying pan. Fry the onion until soft and brown, then stir in the ginger. Add the chickpeas, sugar, coriander, cumin, chilli powder, garam masala and a pinch of salt. Stir, then add the tamarind and tomato and simmer for 2-3 minutes. Add 500 ml (2 cups) water, bring to the boil and cook until the sauce has thickened. Stir in the coriander leaves. Serve with rotis.