Ingredients:

• 1 pound thick beef tenderloin
• Kosher salt and freshly ground black pepper
• 2 tablespoons extra virgin olive oil
• 2 tablespoons yellow mustard (like Coleman's Original English Mustard)
• 1 pound mushrooms (see recipe note)
• 4 thin slices ham (Parma ham if you can get it) or prosciutto
• 1 (7 to 8 1/2-ounce sheet) puff pastry, thawed (see recipe note)
• 2 large egg yolks, beaten
• Flaky salt, for sprinkling on top