Pithivier of Prawns

This is a very elegant starter, not overly difficult to make but it will take some time. First the pastry needs to be made at least one day in advance; the mousse could also be made a day before you need it. Then once the whole is assembled it needs to rest. I specify whiting in the recipe for the mousse but it must be fresh; if it has been previously frozen then it will not work. Do not try to substitute it with another flaky fish but you could use any of the firm fish, e.g. brill, turbot,