For the crab salad, wash the crabs thoroughly and drain. Mix mustard with salt, flour and the remaining ingredients except for the cream, boil in a bain-marie while stirring constantly to a thick sauce, let cool.
Fold in the whipped cream. Mix the shrimps with a part of the sauce, let it soak well. Arrange the salad on lettuce leaves and pour the rest of the sauce over it.
Garnish with shrimps.