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Tip the flour, salt, thyme and mustard powder into a food processor and pulse briefly. Add the butter and pulse again to a fine breadcrumb texture. Add half the egg and 1 tbsp cold water and pulse until it comes together into a dough (you can add up to 1-2 tbsp more water if it’s dry). Tip the dough onto a work surface and knead briefly. Wrap and chill for 30 mins.
To make the filling, cook the potato in a pan of boiling water for 8-10 mins. Drain and leave to steam-dry. Heat the oil and butter in a large frying pan over a medium heat, and fry the leeks for 5 mins. Stir in the flour and cook for 2 mins, then add the milk, a little at a time, stirring between each addition to combine. Add the cream, cheese and mustard, and cook for 1-2 mins more until the cheese has melted. Stir in the chives and season. Remove from the heat and leave to cool for 15 mins.
Heat the oven to 220C/200C fan/ gas 8. Gently fold the salmon, potato, haddock and prawns into the creamy leek mixture, then spoon into a 27cm round pie dish. Roll the pastry out on a lightly floured surface to the thickness of a £1 coin. Drape the pastry over the pie, then trim the excess. Crimp the edges, then shape the off-cuts to decorate the top, if you like. Brush with the remaining egg, then bake in the centre of the oven for 35-40 mins until golden. Serve with steamed greens.