Preheat the oven to 400°F (200°C).
Season the salmon fillet with salt and black pepper.
Sauté the spinach until wilted, then mix with cream cheese, lemon zest, and chopped fresh dill.
Roll out the puff pastry sheet on a lightly floured surface.
Spread the spinach and cream cheese mixture evenly over the puff pastry, leaving a border around the edges.
Place the seasoned salmon fillet on top of the spinach mixture.
Fold the puff pastry over the salmon to enclose it completely, sealing the edges.
Brush the puff pastry with egg yolk for a golden finish.
Place the wrapped salmon on a baking sheet lined with parchment paper.
Bake for 25 minutes or until the pastry is golden brown and crisp, and the salmon is cooked through.
Prepare the dill and lemon butter sauce by mixing softened butter with fresh dill and lemon zest.
Serve the Salmon Wellington sliced into rectangular portions, centered on a white round plate, with a small dollop of dill and lemon butter sauce on the side.