Cook the leeks in a pan of lightly salted boiling water for 1 minute until just soft. Drain and cool under cold running water.
Unwrap the pastry on to a lightly greased baking sheet. Use a sharp knife to lightly score a 1 cm (1/2 inch) border all around the pastry, taking care not to cut all the way through. Lightly mark the inside of the pastry with the end of a fork and brush all over the border with egg.
Mix together the remaining egg, the mascarpone and half the blue cheese and spread the mixture over the pastry. Arrange the leeks on top and scatter over the remaining cheese. Cook in a preheated oven, 200°C (400°F), Gas Mark 6, for 20 minutes until the pastry is golden and cooked through.