With the rack in the middle position, preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.
On a floured work surface, roll out the puff pastry into a 10 x 5-inch (25 x 12.5 cm) rectangle. Place the dough on the baking sheet.
Spread the mustard in the centre of the dough, avoiding the edges. Sprinkle the cheese on the mustard. With a pastry brush, cover the edges of the pastry with the beaten egg. Fold the dough in half lengthwise. Press to seal the dough. Brush the surface of the pastry with the remaining beaten egg. Sprinkle with the pecans.
Bake for about 20 minutes or until the pastry is puffed and golden brown. Slice and serve immediately with Vegetable and Fava Bean Soup, if desired.
The cheese will firm up quickly so eat the pastry warm while the cheese is still runny.