Preheat the oven to gas 6, 200°C, fan 180°C. Scatter the leeks on a baking tray, drizzle with olive oil and cook for 15-20 minutes until tender. Remove from the oven and set aside to cool.
Meanwhile, cut the shortcrust pastry in half. Roll out one piece large enough to cut a 25·5cm (10in) disc, using a dinner plate as your guide. Repeat with the other half. Transfer one pastry disc to a lined baking sheet and spread with the tomato paste or tapenade, leaving a 2cm (½in) border.
In a bowl, combine the cooked leeks, the Capricorn Somerset Goat's Cheese, the walnuts and some seasoning. Spread the mixture over the tomato paste or tapenade.
Brush the base border with beaten egg and top with the remaining pastry disc. Fold up the base pastry border, pinching together with the top disc to seal. Make a small hole in the middle of the pie to allow the steam to escape and brush again with the beaten egg. Scatter over some more walnut pieces, if you like. Bake in the oven for 20-25 minutes until crisp and golden.
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