1. Run a potato peeler down the side of the cucumbers to form long ribbons. Put the ribbons in a strainer, sprinkle with salt and toss to combine. Stand for 15 minutes.
2. Rinse cucumber ribbons with cold water, drain and pat dry with paper towel. Put vinegar and sugar in a medium mixing bowl and stir until sugar has dissolved. Add cucumber and toss to combine. Set aside.
3. Put mayonnaise, yoghurt, rind and 1 tablespoon of the lime juice in a bowl and whisk until combined. Set aside. Peel and devein prawns, then slice through middle to flatten.
4. Put 4 slices of bread on a clean board, spread with a small amount of the yoghurt mixture and arrange prawns on top. Drizzle with yoghurt mixture and put another slice of bread on top. Drain cucumber, then arrange on top of prawn sandwiches. Finally, top with the last slice of bread.
5. Use a serrated knife to remove crusts, then carefully cut each sandwich into 3 bars. Serve immediately.
Salting removes excess moisture from cucumber, leaving it soft but crisp. Once pickled and stored in a clean glass jar, cucumber will keep in the fridge for up to three days