Preheat oven to 350°F. Combine butter and terragon, set aside.

Roll out puff pastry, cut into 4 pieces large enough to enclose each fillet.

Season both sides of each fillets with salt and pepper. Spead whole grain mustard on each fillets. Place mustard side down on puff pastry.

Spread terragon butter on each fillet. Fold over dough and enclose fillet. Place seam side down on baking sheet.

Brush with egg wash and bake 20 minutes until golden brown.