1Preheat the oven to 200°C/400°F/Gas 6 and lightly grease a large, non-stick baking tray.
2Steam the leek for 5 minutes or until tender. Leave until cool enough to handle then, using your hands, squeeze out as much water as possible. This will prevent the pastry becoming soggy during baking. Put the leeks in a mixing bowl and stir in the watercress and crème fraiche. Season the mixture generously with salt and pepper.
3Roll out the pastry slightly on a lightly floured work surface into a large rectangle. Lay the salmon lengthways down the centre of the pastry, then spoon the leek mixture over the top in an even layer.
4Make diagonal cuts, 2.5cm/1in wide, in the pastry either side of the salmon; if you cut away the pastry corners, this makes it easier to plait. Begin to plait the pastry taking one of the strips and folding it over the filling then taking one from the other side and folding that over. Continue until you have plaited the pastry all the way down to make a long log shape.
5Using 2 spatulas, carefully transfer the salmon parcel to the prepared baking tray. Glaze the top with egg and bake for 30–35 minutes until golden and the pastry is cooked through.
6Meanwhile, mix together the mayonnaise, lemon juice and 1 tablespoon water to loosen the mixture, then stir in the herbs. Serve the salmon in slices with a good spoonful of the mayonnaise by the side.