Heat the oven to 200°C (400°F) Gas 6.

Melt the butter in a large saucepan and add the leeks, stir and cover. Cook over very gentle heat for about 15 minutes, stirring occasionally until leeks are soft.

Meanwhile, roll out the pastry on a lightly floured surface to a 33 x 23 cm/12 x 9 inch rectangle. Fold the pastry in half lengthways so that it measures 33 x 11 1/2 cm (12 x 4 1/2 inches). Lightly roll over the pastry to expel any air bubbles.

Press the edges of the pastry together well. Prick with a fork all over. Place the pastry strip on a dampened baking sheet and bake in the oven for 10 minutes.

Beat the egg lightly with a pinch of salt. Remove the pastry from the oven and brush with the egg, pressing the pastry down gently as you brush. Return to the oven for a further 10 minutes until golden.

Add a little nutmeg to the cooked leeks, stir in the cream and season to taste with salt and pepper. Keep the leeks warm, but do not boil.

Carefully lift the pastry from the baking sheet and place on a serving plate. Spoon the leek mixture on top of the pastry and serve at once.