1
Pulse plain flour, fine salt, thyme and English mustard powder in a kitchen processor and briefly pulse. Add the unsalted butter and pulse again until it forms fine crumbs. Add half an egg and 1 tbsp cold water and pulse until a dough forms (add another 1‑2 tbsp water if the dough is dry). Turn the dough onto a work surface and briefly knead. Wrap and chill for 30 minutes.
2
For the filling, boil the potato in a pot of boiling water for 8‑10 minutes. Drain and let it steam. Heat olive oil and butter in a large skillet over medium heat and sauté the leeks for 5 minutes. Add plain flour and cook for 2 minutes, then gradually pour in whole milk, stirring after each addition. Add double cream, aged cheddar and Dijon mustard, cook for another 1‑2 minutes until the cheese melts. Stir in the chives and season. Remove from heat and cool for 15 minutes.
3
Preheat the oven to 220 °C/200 °C with fan/gas 8. Gently fold salmon fillet, potato, smoked hake and king prawns into the creamy leek mixture, then spoon into a round 27 cm pie tin. Roll out the pastry on a lightly floured surface to the thickness of a one‑pound coin. Cover the pastry, trim the excess. Decorate the edges, and shape any leftover dough for a top garnish if desired. Brush with the remaining egg, then bake in the centre of the oven for 35‑40 minutes until golden. Serve with steamed greens.

Source: bbcgoodfood.com

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