Place a rack in the center of your oven and preheat the oven to 400 degrees 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, nonstick 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 puff pastry on a lightly floured work surface and roll it into a 12×14-inch rectangle. With a sharp knife, cut the puff pastry in half crosswise so you are left with two rectangles that are 12 x 7 inches each.
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.
Fold one side of the puff pastry over the top of the fillet like a flap, so that it partially covers it, then brush the top of the flap with egg wash. Fold the next side, adhering it to the first if the flaps are large enough to touch , then brush its top with egg wash. Repeat until all 4 sides are folded over. Carefully flip the salmon over and transfer it to the prepared baking sheet, with the folded side underneath.
Brush the tops of the puff pastry with egg wash, then with a sharp knife, score shallow, diagonal lines in each direction in a crosshatch pattern.
Bake the salmon for 20 to 25 minutes, or until the pastry turns golden brown. Let cool a few minutes on the baking sheet. Cut into four equal pieces to serve.