Grind the coriander, cumin and fenugreek seeds in a spice mill or coffee grinder. In a large wide saucepan, fry the onion, ginger and garlic in 2 tbsp oil until soft, then stir in the ground spices, chillies and turmeric and fry until golden. Add the vegetables and enough water to cover them, season, then leave to gently bubble until cooked through. Meanwhile, heat 2 tbsp of the oil in another pan and fry the mustard seeds and curry leaves for a few seconds and add them, with the oil, to the pot when the vegetables are almost cooked. Stir the coconut milk into the now reduced and heavily spiced water and bring to a gentle bubble, then add about 2 tbsp of the tamarind water. Taste and add more salt, pepper and tamarind to suit. Nestle the tomato quarters on top and leave to simmer for a further 2 minutes. The result should be velvety and soup-like.