How to Make It
Heat oven to 375° F. Line a baking sheet with parchment paper or aluminum foil and set aside.
On a lightly floured surface, roll out a sheet of puff pastry to ⅛ inch thick. Place half the spinach leaves lengthwise down the center of the pastry. Season the fish fillets with salt and place them on top of the spinach. Spread the mushroom tapenade over the fish and top with the remaining spinach. Wrap the fish in the pastry, folding in the sides to seal completely. Place on baking sheet and brush with the egg. Bake until the pastry is golden brown, about 25 to 30 minutes. Serve immediately.