Sprinkle salt on both sides of the salmon. Mix the thyme, oregano, basil, dill, mustard and mayonnaise in a small bowl and spread over the salmon, then sprinkle the Feta cheese on top. Layer the spinach over the Feta cheese.
Roll the pastry out wide enough and long enough to wrap around the salmon, about 5mm thickness. Place the salmon in the centre of the pastry and fold the pastry over the salmon. Place the roll seam-side down on a baking tray. Cut several small slits in the pastry to allow steam to escape. Brush the egg white onto the pastry.
Bake in the preheated oven until the pastry is golden brown and the salmon flakes easily with a fork, about 45 minutes.