Boil beans. Drain when cooked.
Cook onions in ½ pint vinegar.
Put all in one pan and add the rest of the vinegar and boil for 15 minutes.
Mix turmeric, mustard and cornflour with a little vinegar, add to mixture and add the sugar.
Boil for 10 minutes.
Put into jars.
Put the jars on a tray & put in the cold oven then set it on a low heat (I set it to 140c in a conventional oven); this prevents the jars from cracking or breaking when you put the hot chutney in them.
When you add the sugar, stir with a wooden spoon until you are sure the sugar has dissolved before bringing the mixture to the boil. You can tell when the sugar has dissolved if you tap the spoon on the bottom of the pan, if it sounds or feels gritty the it is not quite all dissolved.