Fish pie with prawns and watercress

Method

1. Melt the butter in a pan, add the flour and stir until smooth. Cook for 2 minutes, add a little milk and stir until smooth. Gradually add the rest of the milk, stirring, then add the bay leaves. Simmer for 5 minutes, remove from the heat, season and stir in the mustard and watercress.
2. Scatter the prawns, salmon and frozen peas in a 1.5-litre pie dish. Pour over the sauce and leave to cool completely.
3. On a lightly floured surface, roll out the pastry so it’s larger than the pie dish, then cut into 2cm-thick long strips. Brush the rim of the dish with egg, then place half the strips, at intervals, all going in one direction. Cross with the remaining strips to make a lattice, lifting and weaving them in and out of each other – this is fiddly.
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5. Cover with cling film and foil and freeze, uncooked, for up to 1 month.
6. When ready to eat, remove the foil and cling film and defrost fully, then preheat the oven to 200°C/fan180°C/gas 6. Brush the top with beaten egg, then bake for 25 minutes until the pastry is golden brown on top and the sauce is bubbling.