Prepare the crayfish butter:
In a frying pan, lightly heat the crayfish carcass for a few minutes over medium heat. To release their full aroma, crush them with a pestle.
Stir the butter into the pan with the carcass. Melt the butter over a low heat, stirring to allow the carcass flavors to diffuse into the butter. Remove from heat, cover and leave to infuse for 2 h or more.
Strain the mixture through a fine sieve to remove the carcass, then filter a second time through a sieve to collect a residue-free butter.
Remove the quantity for the recipe and store the rest in the fridge for other recipes.
This crayfish butter is the key to the unique flavor of Nantua sauce.
Prepare the classic béchamel sauce, and towards the end of cooking, add 100 ml (3.4 oz) of heavy cream. Leave to bubble a few times to restore the creamy texture of the bechamel.
Off the heat, incorporate 100 g (3.5 oz) of crayfish butter. Mix well and adjust the seasoning.