Peel the cucumber with a vegetable peeler, halve it lengthways and
remove any seeds with a teaspoon.
Cut the cucumber into thin slices, sprinkle thoroughly with salt
and set aside for 5 minutes.
Rinse to remove salt, and pat dry with paper towels.
Place the prawns in a pan of lightly salted boiling water and simmer
for 2 minutes, or until just cooked.
Drain the prawns and plunge them into cold water.
When the prawns are cool, peel and de-vein them, leaving the tails
intact.
Place the vinegar, sugar, soy sauce and ginger in a large bowl and
stir until the sugar dissolves.
Add the prawns and cucumber, cover and marinade in the refrigerator
for 1 hour.
Toast the sesame seeds in a dry pan over medium heat for 3 to 4
minutes, shaking the pan gently, until the seeds are golden brown;
remove the seeds from the pan at once to prevent burning.
Drain the prawns and cucumber from the marinade. Arrange on serving
plates, sprinkle with the sesame seeds and serve.