Preheat oven to 400°F (200°C).
Season the salmon fillet with salt and black pepper.
Wilt the spinach in a pan over medium heat until just softened; drain any excess liquid.
Mix the wilted spinach with cream cheese, chopped fresh dill, salt, and pepper to taste.
Roll out the puff pastry sheet on a floured surface.
Spread a thin layer of Dijon mustard over the salmon fillet.
Place the salmon in the center of the puff pastry.
Spread the spinach and cream cheese mixture evenly over the salmon.
Fold the puff pastry over the salmon, sealing all edges.
Brush the pastry with egg yolk to help it brown.
Place the wrapped salmon on a baking tray lined with parchment paper.
Bake for 20-25 minutes or until the pastry is golden brown and crisp.
Let rest for 5 minutes before slicing.
Slice into thick portions for serving.
Drizzle dill and mustard sauce artistically around each plate and garnish with fresh dill sprigs.