Preheat your oven to 400 F ( 200 C). Line a large baking sheet with parchment paper. Set aside.
Make the spinach filling. Melt butter in a large skillet over medium high heat.
Add the chopped onion and cook until translucent. Then, add the minced garlic and cook until fragrant, about 1 minute.
Increase the heat to high and add the chicken stock (or white wine) to deglaze the pan. Cook for about 5 minutes, then add the spinach and cook until the spinach softens.
Add in the cream cheese, Parmesan cheese and bread crumbs. Season with salt and pepper to your liking. Mix everything together and remove from the heat.
Remove the puff pastry from the refrigerator, unfold it and cut each sheet in half to make 4 pieces ( or size that's good to accommodate the salmon and the filling). My sheets were about 10x12 inches.
Season the salmon with salt, pepper and drizzle with lemon juice.
Place each seasoned salmon fillet in the middle of the prepared puff pastry sheet. If the sheet is not big enough to fit the salmon and the filling, roll it out a little more. Leave 2 inches around the edges.
Divide the spinach filling between the salmon pieces and spread it on top.
In a small bowl mix together the egg and 1 teaspoon water ( or milk) to make the egg wash.
Brush the edges of the puff pastry with the egg wash.
Close up the sides of the puff pastry pinching and sealing around the edges. Cut off any excess on the ends and flip them over.
Score the tops ( make the crosshatch slits), brush with the egg wash and sprinkle with flaky salt.
Bake in the preheated oven to 20-25 minutes or until puff pastry is golden brown.