Cucumber slices boiled briefly in vinegar, sugar and mustard seeds, then kept in their syrup in the fridge. Twelve minutes of work, and they are ready once cold.
These are quick pickles, not preserves. There is no sterilising and no vacuum seal, so they live in the fridge and keep about three weeks rather than sitting in a cupboard for a year. Thin slices take on the flavour faster and are ready as soon as they are cold; thick ones benefit from a day. Five minutes of boiling is the outer limit before the cucumber goes limp, so take them off on time. Cider vinegar gives a rounder, slightly sweet pickle and white vinegar a sharper one, and both are correct.