Prepare the cucumber by using a vegetable peeler, peeling thin ribbons of flesh and skin, but not the seeds. Place the cucumber ribbons in a sieve over a bowl and sprinkle with salt. Leave for an hour. Later, rinse under cold water and pat dry.
Make the dressing by mixing all the ingredients together, then check the seasoning.
Mix the crab meat with the mayonnaise, crème fraîche, lemon juice, salt and pepper. Taste and adjust the seasoning if necessary.
To assemble: toss the cucumber ribbons in the dressing, with the coriander and red onion and divide between eight plates. Stack three slices of Melba toast on each plate and pile the crab meat into the centre of two Melba toasts.