Preheat the oven to 400°F.

On a piece of parchment paper, roll out the puff pastry 1 to 2 inches longer than the length and more than twice the width of the salmon fillet. Slide the pastry and paper onto a baking sheet, cover, and refrigerate.

Heat the oil in a sauté pan over high heat. Add the spinach and mushrooms and quickly sauté until the mushrooms are a little browned and the spinach has shrunk a bit in size. Whisk together the mirin, miso, and brown sugar and stir into the spinach and mush- rooms. Remove from the heat and let cool.

Place the fish fillet on the dough rectangle, positioning it 3 inches in from one edge. Spread the spinach mixture evenly over the fish. Fold the dough over the fish (like a book) and tuck the top edges under the bottom edges, using your fingertips to seal the dough and making sure the seam is on the underside of the packet. Use a sharp knife to make about 10 slits in the top of the pastry. Brush the top with the beaten egg. Bake until the pastry is golden brown, about 40 minutes. To check the doneness, carefully turn the pastry over and cut a slit into the underside to peek in and check that the fish is done. If it isn’t, return to the oven for a few minutes.

Meanwhile, prepare the miso sauce: Whisk together the mirin, miso, and brown sugar until well blended. Keep at room temperature or warm on the stovetop before serving.

To serve, cut the salmon crosswise into 8 to 10 slices, drizzle each with the miso sauce, and serve.

Source: The Modern Menu Cookbook Photos by Andrew Zuckerman