Preheat oven to 400°F. Line a rimmed baking sheet with parchment paper.
Melt the butter in a small bowl or saucepan. Stir in the Dijon, lemon zest, ¼ teaspoon kosher salt, and ¼ teaspoon black pepper, garlic powder and oregano. Set aside.
Heat the oil in a large, non-stick skillet. Add garlic and cook until fragrant approx. 2 minutes. Then add the spinach in large handfuls, cooking until it wilts, about 1 minute. Stir in the dill and season with salt and pepper to taste. Stir in the feta. Remove from the heat and let cool.
In a small bowl, lightly beat the egg yolk with 1 teaspoon water to act as an egg wash. Set aside.
Place the thawed puff pastry on a lightly floured work surface and roll it into a 12×14-inch rectangle. Remove the salmon fillet from the refrigerator and place on a plate. Brush the top of the fillet liberally with the butter mixture, then place the fillet, butter-side down, in the center of the piece of puff pastry. Spoon the cooled spinach mixture over the top.
