Mix all the ingredients for the dressing and season to taste with salt and pepper.
Rinse the prawns and crayfish tails in a sieve and remove any stray bits of shell. Pat them dry with plenty of kitchen paper, – it is important that they are completely dry, otherwise the dressing will turn thin.
Fold the prawns and crayfish tails into the dressing and set aside.
Cut a slit along the top of each croissant and fill them with the seafood salad.
Top with fresh greens and a little pepper if you like, and serve straight away.