1. Melt the butter in a saucepan over low heat. Add the leeks and cook for 5 minutes until tender. Sprinkle in the flour and stir. Stir in the stock and bring to a boil, stirring often. Remove from the heat and stir in the chicken, thyme, and lemon juice. Season well with salt and pepper. Cover with plastic wrap and let cool.

2. Preheat the oven to 425°F (220°C). Dampen a large baking sheet. Roll out one sheet of the puff pastry on a lightly floured surface. Trim into a 10 × 6in (30 × 15cm) rectangle. Place the pastry on the baking sheet. Roll out and trim the remaining pastry to 10 × 7in (30 × 18cm) rectangle. Lightly dust it with flour, then fold in half lengthwise. Make cuts ½ in (1cm) apart along the folded edge to within 1in (2.5cm) of the outer edge.

3. Spoon evenly over the puff pastry base, leaving a 1in (2.5cm) border. Dampen the edges of the pastry with water. Place the second piece of pastry on top and press the edges together to seal; trim off the excess. Brush with beaten egg. Bake for 25 minutes or until golden-brown and crisp. Cool briefly, then slice and serve hot.