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Trim the ends of the beans and gently pull away any stringy bits on the sides. Cut diagonally into ½cm pieces. Bring a large pan of water to the boil and cook the beans for around 3 mins to soften slightly. Drain and plunge into a bowl of iced water, leave to cool for 5 mins, then drain and set aside.
Put the pan back on the hob and cook the onions in 350ml of the vinegar over a medium-low heat for 15 mins to soften, stirring occasionally. Add the sugar and 250ml of the remaining vinegar and cook for 2 mins more.
Combine the turmeric, mustard seeds and powder and nigella seeds with the cornflour, a good pinch of salt and the remaining 100ml vinegar. Mix well, then tip into the onion mixture and stir to combine. Cook for 3 mins to thicken, then add the beans. Bring to the boil, then turn the heat down and simmer for 30 mins, stirring regularly until the beans are tender but have a slight bite and the chutney has thickened slightly. Remove from the heat and decant into sterilised jars. Will keep sealed in a cool, dry place for up to three months. Once open, keep in the fridge for up to two weeks.