Heat the oven to 425ºF.

Melt 1 tablespoon unsalted butter in a large skillet over medium heat. Add 1 finely diced small shallot and cook until softened, 1 to 2 minutes. Stir in 2 minced garlic cloves and ¼ teaspoon kosher salt, and cook until the garlic is fragrant, about 30 seconds.

Stir in 5 ounces baby spinach a handful at a time and cook until just wilted and vibrant green, 1 to 2 minutes. Stir in 2 ounces room-temperature diced cream cheese until melted and combined with the spinach, 1 to 2 minutes.

Remove the pan from the heat. Stir in 1 ounce finely grated Parmesan cheese, ¼ cup panko breadcrumbs, 2 tablespoons finely chopped fresh dill, and 1 tablespoon finely grated lemon zest.

Pat 1 (1-pound) piece skinless salmon dry with paper towels. Season all over with ½ teaspoon kosher salt and ¼ teaspoon black pepper.

Place a piece of parchment paper large enough to line a rimmed baking sheet on the work surface. Dust with all-purpose flour. Unfold 1 (about 14-ounce) package thawed puff pastry onto it with a shorter side closer to you. (If your pastry comes in two sheets, place them side by side, pinch together to form one sheet, and arrange a shorter side closer to you.) If needed, roll the pastry out from side to side (do not roll from top to bottom) into a rectangle that is about 4 inches wider than the salmon.

Place the salmon across the center of the pastry. Spoon the spinach mixture evenly over the salmon. Brush the edges of the pastry with a thin layer of beaten egg.

Fold the left and right sides to the center. Fold the top and bottom to the center. Trim any excess pastry if necessary. Press the seams to seal the pastry completely around the salmon.

Flip the pastry-wrapped salmon (so the spinach mixture is now on the bottom). Place the parchment paper and salmon on the baking sheet.

Using a paring knife, gently score the top of the pastry with a crosshatch pattern without cutting all the way through the pastry. Brush the top and sides lightly with the beaten egg.

Bake until the center of the salmon registers at least 120ºF and the puff pastry is lightly golden-brown, about 20 minutes. If you’d like more browning on the pastry, set the oven to broil and broil until the pastry is golden-brown, 1 to 2 minutes. Let cool for 5 minutes on the baking sheet before slicing with a serrated knife and serving.