Preheat the oven to 400°F (200°C). Line a baking sheet with parchment paper.

In a skillet over medium heat, melt the butter. Add the diced onion and sauté until softened, about 3-4 minutes. Add the minced garlic and cook for another 1 minute.

Pour in the white wine and cook until most of the liquid has evaporated, about 2 minutes.

Reduce the heat to low and stir in the cream cheese until melted and smooth. Add the baby spinach and cook until wilted, about 1-2 minutes. Remove from heat and stir in the Parmesan cheese, breadcrumbs, salt, and black pepper. Let the mixture cool slightly.

Roll out the thawed puff pastry on a lightly floured surface and cut it into 4 equal rectangles.

Divide the spinach mixture evenly among the centers of the puff pastry rectangles.

Place a salmon fillet on top of the spinach mixture in the center of each rectangle. Season the salmon lightly with salt and pepper.

Brush the edges of the puff pastry rectangles with the egg wash (whisk together the egg and milk). Fold the pastry over the salmon, sealing the edges tightly. Flip the wrapped salmon over so the seam is on the bottom and place it seam-side down on the prepared baking sheet.

Using a sharp paring knife, lightly score the top of each pastry in a crosshatch pattern, being careful not to cut all the way through the dough.

Brush the tops and sides of the pastry with the egg wash, ensuring even coverage. Sprinkle a little extra salt over the tops of the pastries.

Bake for 25-30 minutes, or until the pastry is golden brown and puffed.

Remove from the oven and let rest for 5 minutes before serving. Enjoy!