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

In a bowl, mix room temperature cream cheese, thawed and drained spinach, and shredded mozzarella cheese. Season with salt and pepper to taste. Set aside.

On a 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, centered within the puff pastry. Season with salt and pepper.

Carefully fold the puff pastry over the salmon, sealing the edges firmly. Use water if needed.

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

Make slits on the pastry's top to allow steam to escape during baking.

In a small bowl, beat the egg to create an egg wash. Brush it over the pastry for a golden finish.

Bake for 25-30 minutes or until the pastry is golden brown, and the salmon reaches 145 degrees Fahrenheit (63 degrees Celsius) internally.

Remove from the oven, let it rest for a few minutes, then slice and serve your delightful Salmon Wellington. Enjoy!