1. Roll out the pastry to a strip measuring 46 x 10 cm (18 x 4 inches). Cut into two portions, one 5 cm (2 inches) shorter than the other.
2. Roll out the smaller piece to the same size as the other and place it on the dampened baking sheet.
3. Brush a border 1 cm (1/2 inch) wide up each side of the strip with beaten egg. Spread the jam over the centre of the pastry.
4. Fold the thicker piece of pastry in half lengthways. Using a sharp knife, cut across the fold at intervals to within 1 cm (1/2 inch) of the other edge. Unfold the pastry and lift it carefully on to the portion on the baking sheet. Press the edges well together and knock up with a knife.
5. Bake in the oven at 220°C (425°F) mark 7 for about 20 minutes until golden brown.
6. Remove from the oven, brush with egg white and dredge with caster sugar. Return to the oven and cook for a further 5 minutes to 'frost' the top.
7. Serve hot or cold.