Get Ready: Preheat your oven to a festive 425°F (220°C). On a clean surface, lightly dust with flour and roll out your thawed puff pastry sheet until it's noticeably thinner – this helps it cook evenly and creates a delicate crust! Cut the rolled pastry in half to make two rectangular pieces.
Whip Up the Healthy Spinach Filling: Heat a skillet over medium heat and melt the butter with the olive oil. Add the minced shallot and sauté for just 1-2 minutes until it's fragrant. Stir in the minced garlic and cook for another 30 seconds to 1 minute. Toss in the fresh baby spinach and quickly cover the skillet to help it wilt down.
Flavor Boost: Remove the lid and stir in the fresh dill, bright lemon zest, lemon juice, nutmeg, salt, and pepper. Mix well. Now, add the softened light cream cheese and use your spatula to break it down and melt it into the spinach mixture until it's smooth and creamy. Take it off the heat and let it cool a little while you prepare the salmon.
Assemble Your Wellingtons: Pat your salmon fillets very, very dry with a paper towel – this is key for a crispy pastry! Place one salmon fillet on each of your rolled-out puff pastry halves. Brush 1 tablespoon of Dijon mustard over each fillet. Now, divide the delicious, cooled spinach mixture and spoon it evenly over each salmon piece.
Seal the Deal: Gently fold the sides of the puff pastry over the salmon and spinach, creating a neat little package. Make sure the edges are sealed well. Carefully transfer your two salmon parcels, seam-side down, onto a baking sheet lined with parchment paper.
Golden Perfection: Using a small paring knife, lightly score a decorative design on top of each pastry – just don't cut all the way through! Whisk your egg with a splash of water to make an egg wash, then generously brush it all over the top and sides of the puff pastry. Sprinkle a little coarse sea salt over the top for a beautiful, golden-brown finish.
Bake to Impress: Bake your salmon wellingtons in the preheated oven for 25 minutes. If the pastry starts to look too dark, simply tent it loosely with aluminum foil and continue baking for another 10 minutes, or until the pastry is beautifully golden and the salmon is cooked through (internal temperature of 145°F).
Rest & Enjoy: Once baked, remove from the oven and let your wellingtons rest for 5 minutes before slicing into them. Serve warm with extra lemon wedges for a fresh, healthy pop of flavor!