Soak the brains in cold water for four hours, changing the water frequently. Trim and cut into small pieces. Now crush the garlic, finely mince the ginger and mix with the chilli powder and turmeric. Slice the onions and chop the tomatoes. Heat some oil in a saucepan and fry the onions until golden brown. Sprinkle with a little water, cover and cook until soft. Mash the onions, season with salt and add the tomatoes. Cook, covered, for another five minutes then crush the tomatoes. Add the garlic, ginger, chilli powder, turmeric and garam masala then cook for 10 minutes. Add the pieces of brain care¬fully turning them over once or twice. Add the whole green chillis, coriander leaves, and a little water if necessary. Simmer on a low heat for 25 minutes. The gravy should be fairly thick. Sprinkle with eurnmin and mustard seeds. Serve with chappatis or nan and yoghurt.