Preheat the oven to 400°F (200°C).
Wilt the spinach in a dry skillet over medium heat, then drain and squeeze out excess moisture.
Mix the wilted spinach with cream cheese, chopped dill, and lemon zest until combined.
Roll out the puff pastry sheet on a lightly floured surface.
Spread the spinach and cream cheese mixture evenly over the center of the puff pastry.
Place the salmon fillet on top of the spinach layer.
Fold the puff pastry over the salmon to fully encase it, sealing edges with egg wash.
Brush the outside of the pastry with egg wash for a golden finish.
Place the wrapped salmon seam side down on a baking sheet lined with parchment paper.
Bake for 20-25 minutes or until the pastry is golden brown and crisp, and salmon is cooked through.
Serve a single rectangular slice centrally on a white round plate with a small dollop of dill and lemon butter sauce on the side.