- Preheat the oven to 400 °F (200 °C).
- Unroll the pastry on a lightly floured surface.
- Trim the pastry so that you are left with a rectangle approximately 23 x 25.5 cm.
- Roll out the larger piece of pastry to a 30 cm square, making sure you use a very even, gentle pressure on the rolling pin so that you end up with nice even edges.
- Grease a baking sheet and use the rolling pin to transfer the pastry to the baking sheet.
- Prick the surface of the pastry with a fork. Brush the pastry all over with the beaten egg.
- Place the baking sheet on a high shelf in the oven and bake for 12–15 minutes until crisp and golden.
- To get it extra crisp, heat the oven to its highest setting, sift 1 teaspoon of the icing sugar all over the pastry, and flash it under the grill.
- Remove from the grill and, using a sharp knife, cut the square into three equal strips.
- Turn the strips over, sift the rest of the icing sugar over them, and then flash them under the grill again.
- Cool on a wire rack, then store in a polythene box, each layer separated with greaseproof paper.
- Place the egg yolks and whole egg into a roomy bowl.
- Use an electric mixer to whip the eggs together with the castor sugar until the mixture is pale, thick and creamy (4–5 minutes). Add the milk as you mix.
- Sift the flour onto the mixture and whisk again until smooth.
- Add the lemon zest and juice.
- Place a pastry strip on the plate, and cover with half the lemon filling.
- Put the second pastry strip on the creme, and cover with the remaining lemon filling.
- Finally, top with the final pastry strip.