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Peel the potatoes, cut them into 3cm chunks, and rinse under cold running water for 2 mins. Bring a large pan of salted water to the boil, add the potatoes and simmer, uncovered, until tender. Drain and tip the potatoes onto a cooling rack. Leave to cool, then chill them for 30 mins - this will dry the surface.
Mix the onion with the ginger, green chillies, cumin, chaat masala, chilli powder and black pepper. Add the lemon juice and set aside.
Heat the oil in a wok or karahi to 180C, or when a cube of bread browns in 30 seconds. Deep-fry the potatoes in batches until golden, drain on kitchen paper, and season with salt.
Mix the warm potatoes with the onion and chilli mixture. Drizzle with date and tamarind sauce, spoon over the coriander relish, and sprinkle with coriander and pomegranate seeds. Serve straight away.