Fumet de Poisson (Fish Stock)

Unlike meat-based stocks, fish stock is never simmered for more than half an hour, otherwise the fish bones break down and make the stock bitter. The ingredients are never expensive, but should be heads, trimmings and bones of white fish like cod, whiting or brill, or a mixture. Any fishmonger will be pleased to give you these. This recipe makes about 1.5 litres/2½ pt.