1. Preheat the oven to 400°F (200°C).
2. Prepare the filling by sautéing spinach with minced garlic until wilted, then mix with cream cheese, dill, and lemon zest.
3. Roll out the puff pastry on a floured surface to fit around the salmon fillets.
4. Season the salmon fillets with salt, pepper, and a thin layer of Dijon mustard.
5. Spread the spinach and cream cheese mixture evenly over the puff pastry.
6. Place the seasoned salmon fillets on top of the filling.
7. Wrap the puff pastry around the salmon, sealing the edges tightly to prevent leaks.
8. Place the wrapped salmon seam-side down on a baking sheet lined with parchment paper.
9. Brush the top with an egg wash for a shiny, golden finish.
10. Score the top of the pastry lightly to allow steam to escape during baking.
11. Bake in the preheated oven for 20-25 minutes until the pastry is golden brown and crispy.
12. Let the Salmon Wellington rest for a few minutes before slicing.
13. Serve warm with a side of asparagus, green beans, or mashed potatoes.
14. Store any leftovers in the refrigerator for up to 2 days.