Preheat the oven according to puff pastry package instructions.

In a skillet, melt unsalted butter over medium heat. Add Dijon mustard, ¼ teaspoon kosher salt, and ¼ teaspoon black pepper. Stir until combined.

Add dried thyme, olive oil, spinach, and minced garlic to the skillet. Cook until spinach is wilted, then remove from heat.

In a bowl, combine spinach mixture with Parmesan cheese. Let it cool slightly.

Roll out the thawed puff pastry on a floured surface. Cut into an appropriate size for wrapping the salmon.

Place one salmon fillet on the pastry. Spread the spinach mixture over the fillet, then top with the second salmon fillet.

Wrap the pastry around the salmon, sealing the edges with the egg wash made from the egg yolk and water.

Cut slits in the top of the pastry for steam to escape.

Bake according to puff pastry package instructions until golden brown.