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Put the ginger, garlic and four tomatoes into a food processor and blitz until smooth. Set aside.
Put the potatoes in a large saucepan. Cover with cold water and bring to a simmer over a medium heat. Cook for 8-10 mins, or until just tender. Drain and leave to steam dry.
Meanwhile, heat the oil in a large non-stick frying pan over a medium heat. Add the onions and a large pinch of salt and fry for 15 mins, or until the onions are golden and sticky. Add the chillies, mustard seeds, ground coriander, turmeric, cumin and garam masala to the pan and fry for another 2 mins. Tip in the tomato mixture and bring to a gentle simmer, then carefully stir in the potatoes and remaining tomatoes. Season to taste. Gently simmer the Bombay potatoes for a few mins until everything is warmed through, then serve topped with the coriander.