Prepare the filling by making and cooling the béchamel, then mixing it with cream cheese, Dijon mustard, chopped dill, parsley, lemon juice, and zest.
Quickly cook the baby spinach with a little butter and seasoning, strain the excess liquid, let it cool, and add it to the béchamel mixture.
Pat the salmon fillet dry, season with salt and pepper, then sear on both sides for about a minute in a hot frying pan. Let it cool.
On a lightly floured surface, place the first sheet of puff pastry. Put the salmon in the center, spread the cold spinach mixture on top, and brush egg wash around the salmon.
Cover with the second pastry sheet, trim any excess, and seal the edges with a fork. Brush the pastry with egg wash, score for decoration if desired, and chill.
Preheat the oven to 200°C with a baking tray inside. Place the salmon en croûte onto the hot tray lined with parchment paper, then bake for 20-25 minutes until the pastry is golden brown.
Let the salmon rest for 5-10 minutes before slicing and serving.
