Jeremiah Tower’s Cucumber and Crayfish Cream Vichyssoise

• 1 small onion, finely chopped
• 16 fl oz (475 ml) chicken stock

First make the crayfish essence by putting the shells into a food processor and grinding just enough to break them up. Put in the bowl of an electric mixer, fitted with a dough hook or paddle. Add 8 fl oz (250 ml