Preheat the oven to 400 degrees Fahrenheit (200 degrees Celsius).

In a mixing bowl, combine the room temperature cream cheese, thawed and drained spinach, and shredded mozzarella cheese. Mix until all the ingredients are well incorporated. Season this mixture with a pinch of salt and pepper to taste. Set the filling aside.

On a lightly floured surface, roll out the thawed puff pastry sheet into a rectangle.

Place the cream cheese mixture in the center of the puff pastry, leaving some space around the edges.

Lay the salmon fillet on top of the cream cheese mixture, centered within the puff pastry.

Season the salmon with salt and pepper to taste.

Carefully fold the puff pastry over the salmon and filling to encase it completely. Make sure to seal the edges by pressing them together firmly. Use a little water to help seal, if needed.

Transfer the prepared Salmon Wellington to a baking sheet lined with parchment paper, seam side down.

Using a sharp knife, make a few slits on the top of the pastry to allow steam to escape during baking. This will help prevent the pastry from getting soggy.

In a small bowl, beat an egg to create an egg wash. Brush the top of the pastry with the egg wash to give it a nice golden color when baked.

Place the baking sheet in the preheated oven, and bake for approximately 25-30 minutes or until the puff pastry is golden brown and the salmon is cooked to the desired level of doneness. The internal temperature of the salmon should reach 145 degrees Fahrenheit (63 degrees Celsius).

Remove the Salmon Wellington from the oven, and let it rest for a few minutes before slicing and serving.