Prepare a clean pickling jar by sterilising it with hot water. Fill with boiling water, leave for a few minutes, then repeat. Tip away the water.

Put the vinegar, water, salt, sugar and mustard seeds into a saucepan. Bring to the boil, then turn off the heat and leave to cool for a few minutes.

Put the cucumbers and jalapeños in the pickling jar. Pour over the liquid (including all of the seeds). Close the jar. Transfer to the fridge and leave for at least 12 hours before eating for best results.