Slice the onion thinly. Boil the spinach and cut into 2cm pieces.

Boil the salmon in the same pot used for spinach. Remove bones and skin, then cut into bite-sized pieces.

Melt butter in a pan, add spinach and onion, and sauté over medium heat until softened. Add flour and cook until no longer powdery.

Add salmon, milk, salt, and pepper. Stir until thickened, then cool to room temperature and refrigerate.

Roll out the frozen puff pastry sheets and cut each into 4 equal pieces.

Place the filling on one piece of pastry, cover with another, and seal the edges with a fork. Make shallow cuts on top.