Put the cauliflower in a saucepan of boiling water and parboil for 4 minutes, or until tender but retaining a little firmness. Drain and refresh under cold water, then drain again.
Melt the ghee or butter in a wok or a large frying pan over a medium heat then tip in the black mustard seeds. When they start to pop, stir in the onion and curry leaves and fry until the onion starts to colour. Drop in the chillies, turmeric, ginger and garlic. Cook for a further minute, stirring often to prevent burning, then tip in the cauliflower and salt. Finally put in the tomatoes and keep tossing everything over the heat until the tomato has only just softened. Check the seasoning.
Serve garnished with coriander, if you like, then finish with a squeeze of lime.