To prepare the chard, cut the stems from the leaves. Cut the stems into 4cm pieces and slice the leaves into 4cm strips.
Put the oil into a large lidded frying pan and, when hot, add the mustard seeds. When they pop, add the onions and cook for 12 to 15 minutes, until soft and golden brown. In the meantime, peel the potatoes and cut into 2cm cubes.
When the onions are ready, add the ginger and garlic to the pan and cook for a couple of minutes, then add the potatoes and 200ml of water. Cover and cook for 10 minutes.
Add the tomatoes and the chard stalks, cover and cook for a further 5 minutes, or until the chard stalks are soft. Add the chilli, cumin, coriander, turmeric and salt and stir gently.
Finally, add the chard leaves, coat with the mixture and pop the lid back on for a final 2 to 3 minutes, until the leaves have wilted. Serve with hot chapattis or rice, yoghurt and a little pickle.