PREPARATION

Melt a 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.

Meanwhile, preboil the potatoes in soft salted water for 10 minutes, drain, then add to the leek, mix and gently stir through. Let cool and then mix with pecorino cheese and walnuts. Season with salt and pepper.

Preheat the oven to 200°C (fan oven -20°C). Remove the pastry from the fridge and cut in half. Roll the pastry on lightly floured surface to thickness of 2mm and use then to line 20 x 30 cm 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 and lay them over the filling in a criss-cross. Trim the overhanging pastry with a sharp knife, then brush the lattice well with a beaten egg. Slide onto a baking sheet and bake for 25 minutes until golden.

Cool slightly in a thin, then transfer to a board to slice. Serve hot or cold with a glass of cider or white beer.