Put a splash of oil in a large pan with a lid. Turn on the heat and add the mustard seeds and cardamom. When you hear the seeds crackling, remove from the heat and add the onion, turmeric and cumin. Fry for 30 seconds, stirring continuously, then add some water to stop the spices burning.
Add the potatoes and more water to cover them. Simmer for 4 minutes then add the cauliflower and season with salt. After 5 minutes or so the vegetables should be cooked and the sauce thickened. Cook on for another minute or so if the suace is too thin.
Decorate with a few coriander leaves and serve with Peshwari Naan.
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