Preheat the oven to 400℉. Leave the salmon in the fridge
Melt the butter in a small frying pan. Stir in the mustard, lemon zest, ¼ tsp of the salt and half the pepper. Set aside.
Prepare the spinach. If using large leaves, pull off the spine. If using baby spinach, this is not necessary. Carefully wash the spinach and drain in a colander. Heat the oil in a large non stick frying pan. Add the spinach with the leaves still a bit damp to the pan, stirring it until it wilts in a couple of minutes. Stir in the garlic, thyme, remaining ½ tsp of salt and remainder of the pepper. Stir in the Parmesan cheese. Remove from the heat and set aside.
Make an egg wash. Beat the egg in a small bowl with a splash of water. Set aside
Place the puff pastry on a lightly floured surface and measure it. You might wish to roll it a little thinner than it came out of the package. With a sharp knife, cut it into 2 rectangles measuring 12 x 7 inches each.
Remove the salmon portions from the fridge. Pat them dry and then using a pastry brush, brush the mustard, lemon and butter mixture on the top of each portion. Then place each portion, butter side down in the middle of the puff pastry. Spoon half of the spinach mixture over each portion.
With each portion, fold one side of the puff pastry over the top of the fish, then brush with the egg wash. Fold the next side over and repeat with the egg wash, until all four sides have enclosed the salmon portion. The fish should be well sealed. Carefully flip the salmon over and transfer it to a rimmed baking sheet that has been lined with parchment paper.
Brush the tops of the puff pastry with the egg wash, then score shallow diagonal lines into the top in a crosshatch pattern
Bake for 20 to 25 minutes, or until the pastry turns golden brown. Serve immediately.