Drain the crabs well. Stir the remaining ingredients to a dressing (must not be liquid!). Stir in the shrimps and leave to infuse in the refrigerator. If desired, fold in finely chopped scallions and/or chive rolls before serving. Season to taste. Line cocktail glasses with lettuce leaves, pour in crab mayonnaise and garnish.
Our tip: Fresh chives are much more aromatic than dried ones!