Preheat Oven: Preheat oven to 180C fan forced (200C conventional). Line a large oven tray with baking paper.
Cook Onion: Heat oil in a large frying pan on medium high. Cook onion for 3-4 mins, until softened. Season with salt and white pepper.
Add Spinach: Add spinach and cook, stirring for 1-2 mins until wilted.
Prepare Spinach Mixture: Transfer to a large bowl and allow to cool slightly. Pour off any excess liquid. Stir in cream cheese until well combined. Stir in lemon zest and refrigerate until needed.
Prepare Salmon: Trim any excess belly from salmon, then halve widthways into 2 equal sized pieces.
Assemble Salmon: Spread cream cheese mixture on one piece of salmon on topside. Top with other piece of salmon and sandwich together.
Prepare Pastry: Lay 1 sheet of puff pastry on a clean board. Carefully transfer salmon to the centre of the pastry.
Seal Pastry: Brush edges with some egg wash, then top with the other pastry sheet. Press edges to seal, then trim excess pastry around the parcel with a pastry cutter.
Decorate and Bake: Brush top with remaining egg wash, then use a teaspoon to imprint top with a scale pattern. Sprinkle top with some sea salt then bake for 40 mins, until golden and puffed.
Serve: Allow to rest for 5 mins, then slice and serve with lemon wedges and a side salad.