When ready to assemble, lay 1 of the pastry sheets on a flat floured surface. Brush with egg wash and evenly spread the spinach on top. Then cover the spinach with the sauteed mushrooms.
Preheat oven at 350 degrees F.
Place the other pastry sheet on top of the salmon and press the 2 sheets together so they seal tightly (the whole thing should look like a parcel). Egg wash the top and sprinkle with little salt. Transfer the salmon to a sheet pan and place in the preheated oven for 30 minutes.
After 30 minutes check internal temperature by placing an instant-read thermometer into the center of the salmon. When it reads 125 degrees F remove from the oven and allow to rest for a further 10 minutes. It will continue to cook for a further 20 minutes or so.
Transfer salmon to a cutting board and using a serrated knife, slice into 2-inch pieces. Place on a serving platter and enjoy.