Mix the melted butter with mustard, lemon zest, and seasonings. Sauté the spinach and stir in the Parmesan. Roll out the puff pastry and slice it in half crosswise.
Brush each salmon fillet with the butter mixture and place one in the center of each puff pastry sheet.
Top each fillet with the spinach-Parmesan mixture. Fold one side of the pastry over the top and brush with egg wash. Fold the opposite side over and seal, brushing again with egg wash.
Fold in the remaining sides to fully enclose the salmon. Place seam-side down on a parchment-lined baking sheet.
TIP: Seal edges tightly! The egg wash helps bind the pastry.
Brush the tops with egg wash and score lightly with a knife for steam release and a beautiful finish.
Bake at 400°F in the Ninja Woodfire Oven for 20–25 minutes until golden brown. Cool slightly, then enjoy!