Preheat oven to 400 degrees Fahrenheit. Melt butter in a small bowl or saucepan. Add Dijon mustard, lemon zest, 1/4 teaspoon kosher salt, and 1/4 teaspoon black pepper. Set aside.

Heat oil in a large skillet. Add spinach in large handfuls, cooking until it wilts, about one minute. Add garlic, remaining 3/4 teaspoon salt, and 1/4 teaspoon black pepper. Stir in Parmesan cheese. Remove from the heat and let cool.

In a small bowl, lightly beat egg yolk with one teaspoon water for the egg wash. Set aside.

Place puff pastry dough on a lightly floured work surface and roll out into two large rectangles, about 12×10 inches.

Slice salmon in half width-wise. Brush salmon liberally with butter mixture, then place one piece of salmon onto one sheet of puff pastry. Spoon spinach mixture over salmon, then lay the second piece of salmon over the bottom piece.

Drape the second sheet of puff pastry over salmon. Seal pastry around salmon and trim any excess. Crimp the edges with your fingers.

Brush puff pastry with egg wash, then use a sharp knife to score shallow, diagonal lines in each direction in a crosshatch pattern (as shown), being careful not to cut all the way through. Bake for 35 to 40 minutes, or until a skewer poked in the middle comes out piping hot. Let rest for five minutes before cutting.