Begin with the cream cheese filling.
Finely dice shallot and fry it in the butter, before adding the spinach.
Cook until the moisture has gone, before allowing it to cool.
Place your cream cheese into a muslin cloth and squeeze out all the excess water.
Add the cream cheese, spinach and shallot to a food processor and blend until smooth and all incorporated. Season to taste with salt and pepper and set to one side.
Skin the salmon with a sharp knife, and trim to the right size (you can use any leftovers to make fish cakes or fish pie)
Preheat the oven to 200 C / 400 F
Cut a piece of baking paper to fit an oven tray and grease it with butter. Lay one of the puff pastry sheets on top.
Lay the salmon on the puff pastry and cover in the cream cheese and spinach filling.
Brush an egg wash (one egg mixed with a tablespoon of water) around the edges of the pastry and lay the second sheet on top of the fish. Press it down around the salmon and use a fork to crimp the edges together, sealing them.
Using a sharp knife, gently scrape a scale pattern into the top making sure you don't cut too deep.
Brush the egg wash over the whole salmon en croute, and bake in the oven for 30 minutes.Rest for 10 minutes before serving.