Place the fish in a saucepan, pour over the milk and add the flavoring ingredients. Bring to the boil, cover with a lid and simmer for 10-12 minutes, until the fish is almost tender. Skin and bone the fish, then roughly flake into a shallow ovenproof dish. Scatter the prawns over the fish. Strain the milk and reserve.

Melt 50g/2 oz of the butter in a pan. Stir in the flour; cook for 1 minute. Stir in the milk and cream. Bring to the boil, stirring, then simmer for 2-3 minutes, until thickened. Remove from the heat and stir in gruyere, watercress, mustard, and season. Pour the mixture over the fish and leave to cool.

Preheat the oven to 190C, then melt the remaining butter. Brush one sheet of filo pastry with a little butter, then crumple up loosely and place on top of the filling. Repeat with the remaining filo sheets and butter until they are all used up and the pie is completely covered.

Bake in the oven for 20-30 minutes, until the pastry is golden and crisp. Serve immediately.