Leek, potato and gorgonzola tart

Method

1. To make the pastry, put the flour in a mixing bowl and rub in the butter with your fingertips until the mixture resembles breadcrumbs. Add the salt and toss to combine. Gradually add the water, just until it comes together. Shape into a rectangle, wrap in cling film, then chill.
2. Melt the butter for the filling in a large pan over a medium-low heat, then add the leeks and thyme leaves. Season and fry gently, covered, for 10-15 minutes until softened. Remove from the heat.
3. Meanwhile, put the potatoes in a large pan of cold salted water and bring to the boil. Parboil for 10 minutes, drain, then add to the leek mix and gently stir through. Leave to cool completely, then mix through the gorgonzola and walnuts. Season.
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5. Preheat the oven to 200°C/fan 180°C/gas 6. Remove the pastry from the fridge and cut in half. On a lightly floured surface, roll one half out to the thickness of a £1 coin, then use to line a 20cm x 30cm shallow, fluted, loose-bottomed tart tin – it should just drape over the sides. Tip in the cooled filling. Roll out the rest of the pastry, cut into thin strips, then lay them over the filling in a criss-cross (there will be leftovers; see tips). Trim the overhanging pastry with a sharp knife, then brush the lattice well with the beaten egg. Slide onto a baking sheet and bake for 25 minutes until golden. Cool slightly in the tin, then transfer to a board to slice. Serve hot or cold.