Cut the cucumber into four 2-inch lengths, then cut these across the centre, scoop out and chop the centre pulp, making a boat shape.
If wished the peel can be entirely removed or scored with a fork giving a 'stripy' effect.
Season the cucumber and sprinkle with lemon juice or vinegar.
Put the cucumber pulp, hard-boiled egg whites and shrimps into a basin.
Mix with mayonnaise.
Pile into the cucumber boats and garnish with the chopped egg yolk.
Serve on a bed of lettuce.