Step 1: Prepare the Filling

1. Heat a skillet over medium heat and add butter.
2. Sauté the onions until translucent, then add the mushrooms and garlic.
3. Cook for 2-3 minutes, then stir in spinach until wilted.
4. Remove from heat and mix in cream cheese, lemon zest, thyme, salt, and pepper.
5. Allow the mixture to cool completely.

Step 2: Prepare the Salmon

1. Pat the salmon fillets dry and season them with salt, pepper, and a light layer of Dijon mustard.
2. Set aside while you prepare the puff pastry.

Step 3: Roll Out the Puff Pastry

1. Lightly flour your surface and roll out the puff pastry to fit around the salmon.
2. Cut the pastry into two equal portions.

Step 4: Assemble the Wellington

1. Spread the spinach-mushroom filling evenly over the center of each pastry piece.
2. Place the salmon fillet on top of the filling.
3. Wrap the pastry around the salmon, sealing the edges with a bit of water.
4. Brush the top with egg wash for a golden finish.

Step 5: Bake to Perfection

1. Preheat the oven to 400°F (200°C).
2. Place the wrapped salmon on a parchment-lined baking sheet.
3. Bake for 20-25 minutes or until the pastry is golden brown and crispy.