1. Check the salmon for pin bones, removing any that you find with tweezers, then cut in half. Line a baking tray with a lightly oiled piece of foil.
2. For the mushroom duxelles & Spinach: Place a medium sauté pan over medium-high heat. Add oil / when hot, add mushrooms. Season well with salt and pepper, then add thyme. Stirring frequently to prevent mushrooms from sticking, cook until all moisture has evaporated, about 10 minutes. Remove from heat and allow to cool. Using the same pan add your spinach and sauté quickly, seasoning with salt & pepper for 1-2 minutes or until all moisture has evaporated.
3. Pat the salmon fillets dry with paper towel, then season lightly with salt and pepper. Spread the mustard over one fillet, on the boned side.
4. Roll out the pastry (thinly) on a lightly floured surface into a rectangle, achieving the thickness of a .25 cent quarter, and large enough to enclose the salmon. Put the salmon parcel in the center of the pastry and brush the surrounding pastry with egg. Bring up the edges, trimming off any excess, and tuck them in before folding the rest of the pastry over to form a neat parcel. Carefully turn the whole thing over so that the seam is underneath and place on the prepared baking tray.
5. Brush the pastry with beaten egg. Lightly score a herringbone or cross-hatch pattern using the back of a knife. Sprinkle with salt and pepper. Cover loosely and chill for 15 minutes. Meanwhile, heat the oven to 350°F.
6. Bake the salmon for approx. 20-25 minutes, until the pastry is golden brown and crisp. To test if ready, insert a skewer into the middle. It should feel warm for medium cooked salmon. A piping hot skewer indicates that the fish is well done.
7. Rest the salmon for 5 minutes, then cut into portions using a serrated knife. Serve with lemon cream sauce, and your favorite seasonal vegetables.