Clean the crayfish, wash thoroughly under running water and remove the small black intestines under the tails. Pour 4 1/4 pints of water and the pack of court bouillon into a large stewpot. Cook for 15 minutes then plunge in the crayfish. Allow to cook for 5 minutes once the water is boiling again. Then remove the stewpot from the heat and allow the crayfish to cool in the cooking water. Use scissors to shell the still warm crayfish, keep a few heads aside for decoration. Make a mayonnaise with the mustard, salt, pepper, egg yolk, oil and lemon juice. Once the mayonnaise is made, mix in the cream and the ketchup, keep in the refrigerator. Arrange the crayfish on a bed of lettuce, serve with the sauce.
