Directions

1. Preheat the oven to 425 degrees. Line a baking sheet with parchment paper and mist with cooking spray.
   
2. On a lightly floured surface, roll the puff pastry into a 14-inch square and cut into four 7-inch squares. Brush the center of each with mustard and season with salt and pepper.
   
3. Toss the breadcrumbs and cheese in a bowl and sprinkle over the mushrooms. Fold in the pastry edges, pleating to fully cover the mushrooms; press to seal. Brush both sides of the pastry with the beaten egg. Place seam-side down on the baking sheet, make slits in the top and bake until golden and crisp, 25 minutes. Let cool slightly before serving.