Roll out 2 pastry sheets to 1/2-inch thickness and set aside in the refrigerator for 30 minutes to rest prior to assembly. (This will make it easier to handle). At this time preheat the oven to 350 degrees F.
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.
Lay the whole side of salmon on top of the spinach and mushrooms, making sure you have at least 1/2-inch of pastry left all around the salmon.
Preheat oven at 350 degrees F.
Season the salmon with salt and pepper and top with the chopped eggs and sliced raw onion.
Sprinkle with chopped dill and thyme and brush the egg wash liberally around the edges of the bottom pastry.
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.