Peel and chop the onions into small pieces. Slice the beans and boil them with the onions in salted water until both are tender. Strain and drain well and then cut into small pieces or put through a mincer.
Add the sugar and 750ml/1 1/4 pints of the vinegar and boil for 15 minutes.
Mix the turmeric, mustard and cornflour with the remaining vinegar and add this to the pan with everything else.