Preheat oven to 200°C convection.
Heat some olive oil in a pan and sauté the mushrooms for a few minutes, season with salt and pepper. Add the garlic and the spinach. Now continue to fry until the spinach has shrunk down. Remove the pan from heat and add the crumbled feta. Leave to cool in a bowl.
Roll out the puff pastry and place half of the spinach mixture on one half of the pastry (in about the size of the salmon fillet). Place the salmon on top, season it with salt and pepper, and cover the salmon with the other half of the pastry. (You may want to cut down a small amount of pastry for decorating later).
Press the edges shut with a fork. Use the cut dough to decorate the salmon pocket. For example, form small fish and glue them with some beaten egg on it.
Brush the pastry now with beaten egg, sprinkle with (coarse sea) salt and bake in the oven at 200°C for 25 minutes.
Serve with a fresh lemon wedge. Enjoy your meal!