In a medium sauté pan over medium heat sauté shallots for 4-5 minutes, until softened. Add garlic and continue to sauté for an additional minute. Add the spinach, a pinch of salt and then wilt the spinach.
Drain off any excess liquid and add the mixture to a medium size mixing bowl. Add cream cheese, dill, parsley, lemon zest & lemon juice. Mix then place in the fridge to cool.
Preheat the oven at 200c and line a baking tray with baking paper.
Double check the salmon for any bones, if you find any remove them with tweezers. Keeping the salmon boned side up, cut the salmon in half.
Neatly spread the cream cheese mixture over one of the salmon fillets avoiding about 1cm from each edge. Now place the other salmon fillet on top in the opposite direction, so that both ends are of even thickness.
If there is any cream cheese leaking out of the sides, scrape it off and discard. Place the salmon to the side.
Lightly flour a surface and roll out the pastry into a rectangle that is large enough to enclose the salmon and with a thickness of around 2-3mm.
Place the salmon in the middle, season with salt. Brush the edges of the pastry with egg wash.
Fold over the width side of the pastry on top of the salmon and lightly press down the edges. Brush the top edges of the pastry and fold them over. Press down gently on all the edges to seal the pastry. Gently flip the salmon over onto the baking tray.
With the back of your knife lightly score the pastry with a cross-hatch pattern. Lightly brush the pastry with the beaten egg.
Bake the salmon en croute for 15-20 minutes, until the pastry is golden brown and crisp. Test the salmon is cooked by either inserting a metal skewer & checking if it's hot or by using a probe (check the notes for more details)
Sprinkle with a little sea salt & rest for a few minutes. Cut into portions using a serrated knife and serve with your chosen sides.