Heat oven to 200C/fan 180C/gas 6. Put the leeks into a large saucepan with 3 tbsp water and a couple of knobs of butter. Cover, then cook for 10 mins until tender. Remove from the pan, allow to cool, then mix with the crème fraîche and some seasoning.
To assemble the parcels, brush a sheet of pastry with melted butter, then lay 2 more sheets on top, brushing butter between each layer. Place a salmon fillet in the centre, season, then spoon over half the leek mix. Fold the ends over the top, pull up the sides and scrunch together to enclose. Repeat.
Brush both the parcels with melted butter. Put onto a lightly greased baking sheet, then cook for 20-25 mins until browned and crisp. Delicious served with a green salad.