• Heat the oven to 200°C/390˚F. Put the mascarpone or cream cheese in a food processor with the watercress, spinach and rocket and whizz the lot until you have a creamy green puree. Season well.
• Roll out the pastry so that you can completely wrap the salmon in it (about 2-3 mm thick) and place it on a buttered or oiled baking tray (it will hang over the edges). Place the salmon in the centre. If it has a thinner tail end, tuck it under. Spoon half of the watercress mixture over the salmon. Fold the pastry into a neat package (the seam will be at the top, so trim the edges neatly), making sure you don't have any thick lumps of pastry as these won't cook properly. Trim off any excess if necessary. Make 3 neat cuts in the pastry to allow the steam to escape, and use the scraps to make some decorations to hide the joins if you like. Brush with the egg glaze.
• Bake for 30 minutes or until the pastry is crisp and browned. To test wether the salmon is cooked, push a sharp knife through one of the cuts into the flesh, wait for 3 seconds then test it against the inside of your wrist; if it is hot, the salmon is cooked. Serve with the rest of the watercress puree as a sauce.

Short crust pastry

• Sift the flour into a large bowl, add the butter and rub in with your fingertips until the mixture resembles fine breadcrumbs. If you have a food processor you can use that as shown in the above video.
• Stir in the salt, then add 2-3 tbsp of water and mix to a firm dough. Knead the dough briefly and gently on a floured surface. Wrap in cling film and chill while preparing the filling.
• For best results make sure the butter is very cold.