Preheat oven to 400°F (200°C).
In a bowl, mix spinach, cream cheese, lemon zest, and chopped fresh dill. Season with salt and black pepper.
Roll out the puff pastry sheet on a floured surface.
Spread the spinach and cream cheese mixture evenly on the puff pastry, leaving a border around the edges.
Place the salmon fillet on top of the spinach mixture. Season with salt and black pepper.
Fold the puff pastry over the salmon, sealing the edges with beaten egg yolk.
Brush the top with egg yolk for a golden finish.
Bake in the preheated oven for 25-30 minutes or until the pastry is golden brown and crisp, and the salmon is cooked through.
Remove from oven and let rest for 5 minutes before slicing.