Pick through the crab meat to remove any cartilage or pieces of shell.
To make the mayonnaise: Place the egg yolks in a bowl with the mustard, salt and white wine vinegar. Mix together with a whisk. Gradually whisk in the sunflower oil until you have a thick mayonnaise. Check the seasoning and add a pinch of cayenne and a squeeze of lemon juice.
Peel the cucumber and cut in half lengthways. Remove the seeds with a spoon and then thinly slice the cucumber flesh. Season with salt and a little lemon juice.
Slice the bread with a bread knife, allowing 2 slices per person.
Butter the bread and then top one slice with the crab mayonnaise. Add the cucumber slices and place another slice of buttered bread on top. Cut in half or quarters and serve immediately.
«Add some fresh herbs such as coriander or parsley and spice the sandwich up with a little fresh chilli.»