Finely dice the shallot and fry it in the butter over medium heat, before adding the spinach. Cook until the moisture has gone, before allowing it to cool completely.
Place your cream cheese into a muslin cloth and squeeze out all the excess water. Then 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.
Make the salmon en croute
Preheat oven to 200 degrees C / 400 degrees F.
Roll out the puff pastry into two rectangles so that both pieces are slightly larger than the salmon.
Prepare the salmon. Remove the skin carefully with a sharp knife (or fish filleting knife), and trim it if needed (you can use any leftovers to make fish cakes or fish pie).
Line a baking sheet with parchment paper and grease it lightly with butter. Lay one of the puff pastry sheets on top.
Lay the salmon on the puff pastry and spread the cream cheese and spinach filling all over the top, right to the sides.
Brush an egg wash (one egg mixed with a tablespoon of water) around the edges of the bottom puff pastry sheet 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.
Decorate your salmon in puff pastry. Using a sharp knife, gently score 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.
Dill cream sauce
Whilst your salmon en croute is in the oven, make the sauce.
Heat some butter in a pan and fry the diced shallot. Add the white wine and bring it to a simmer, allowing it to cook down by half (around 10-15 minutes).
Turn down the heat and add the cream, before bringing it back to a simmering point. Cook for a further 5-10 minutes, stirring continuously before adding the dijon mustard.
Remove from the heat and add the lemon juice, herbs, salt and pepper just before serving.
Tips to avoid soggy pastry! Blot the salmon thoroughly before use to absorb any excess moisture.Strain the cream cheese through cheesecloth/muslin to get rid of any water content and make sure it’s really thick and dense. I have found that most cream cheeses, bar the expensive organic ones you find in farm shops, have so much water in them and this will complicate things for you, especially if you make the salmon en croute in advance.Squeeze out the spinach once it is cooked, it also holds onto a lot of water!