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Put the onions in a preserving pan with 500ml of the vinegar. Cover and cook gently for 20 mins until soft. Meanwhile, blanch the beans for 5 mins, then drain.
Mix the flour, mustard powder, turmeric and celery salt. Heat the mustard seeds in a small lidded pan until you hear them popping, then tip them into the flour mixture. Gradually stir in remaining vinegar until smooth.
When the onions are ready, tip in the spice vinegar mixture and stir continuously over the heat until thickened. Add the blanched beans and cook, uncovered, for 10 mins, stirring occasionally. Add both the sugars and cook, stirring frequently, for 15 mins more until thick. Spoon into sterilised jars (see tip) and seal while hot. This will keep in a cool place for a year, but the colour will fade over time. Once opened, store in the fridge.