Put the oil into a large lidded pan over a medium heat and, when hot, add the mustard seeds and cumin seeds. Stir for a minute, or until they pop, then throw in the onions. Fry for 10 to 12 minutes, until they turn translucent and start to caramelize, then add the garlic and ginger. Stir-fry for around 3 minutes, then add the tinned tomatoes, pouring them in with one hand and crushing them with the other. Fill the empty tin a third of the way up with water and add that to the pan too. Cook for 10 minutes, until quite dry and paste-like, then add the chickpeas. Warm them for a couple of minutes, then add the coriander, chilli powder, turmeric and salt. Toss the chickpeas around in the paste, and add the spinach – trying to fold it all in will be like pushing a duvet into a magical handbag, but it will wilt and shrink fairly quickly. Cook for around 5 minutes, until the spinach is soft and tender, and serve with chapattis (see page 288 of Fresh India) or basmati rice, and a dollop of yoghurt.