Put the oil into a wide-bottomed, lidded frying pan on a medium heat and, when it's hot, add the mustard seeds. When they start to pop, add the onion and fry for around 6 to 8 minutes, until it is nice and soft.
Add the shredded cabbage and 50ml of warm water, stir and pop the lid on. Allow to cook through for 5 minutes, then add the turmeric, salt, cumin, chilli powder and tomato puree.
Stir, pop the lid on again and cook for another 6 to 8 minutes, until the cabbage is really soft without any crunch left in it.