John Dory with crab juices

• 1 Cornish crab (about 700 g), cooked
• unsalted butter
• mirepoix of 1

Crack open the crab shell and remove all the meat. Pick it over to be sure there are no bits of shell or cartilage, then set aside. Using the back, blunt edge of a large, heavy knife, coarsely crush the crab shell.

Melt a knob of butter in a large saucepan, add the mirepoix and cook over a low heat for about 10 minutes or until softened and lightly coloured. Add the crushed crab shells