Mix all the dressing ingredients together and season to taste with salt and pepper.
Rinse the prawns and crayfish tails in a sieve and pick out any bits of shell or other impurities. Pat them dry with plenty of kitchen paper – it is important that they are completely dry, otherwise the dressing will become watery.
Fold the prawns and crayfish tails into the dressing and keep the salad in the fridge until serving. Top with fresh pea shoots, watercress or similar, if you like.