1.8kg runner beans (weighed after trimming and slicing into thin strips lengthwise)
1.4kg onions (peeled and finely chopped)
1.6L cider vinegar
3 heaped tbsp. toasted mustard seeds
2 tbsp turmeric
1kg demerara sugar
To begin, put the chopped onions into a preserving pan or large casserole or saucepan with 275mL of the vinegar. Bring them up to simmering point and let them simmer gently for about 20 minutes or until the onions are soft.
Meanwhile, cook the sliced beans in boiling salted water for 5 minutes, then strain them in a colander. Add them to the onions once you've shaken off all the water.
Combine the cornflour, mustard and turmeric in a small bowl with a little of the remaining vinegar – enough to make a smooth paste – then add this paste to the onion mixture.
Pour in the rest of the vinegar and simmer together for 10 minutes. Stir in the sugar and molasses (keep stirring until they dissolve) and continue to simmer for a further 15 minutes.
Pot the pickle in warmed, sterilised jars, and seal and label when cold. Keep for at least a month before eating.