Remove the skin and bones from the salmon fillet.
Wash the spinach and onion thoroughly and chop them.
Season the onion and spinach with salt and pepper, then sauté in butter.
Spread the pastry sheet, place the sautéed spinach and onion on it, then add the salmon on top.
Sprinkle lemon juice and a bit of herbs over the salmon.
Wrap the filling with the pastry and seal the edges well.
Brush the pastry surface with beaten egg.
Bake in a preheated oven at 180°C for 20-25 minutes.