1. Preheat the oven to 400oF. Pat the tenderloin dry with paper towels and sprinkle with salt (in moderation) and pepper. Heat 1 tablespoon butter in a frying pan and brown meat on all sides. Let cool on a plate and wipe out pan with paper towels.
2. Heat the remaining butter in the frying pan and cook the mushrooms over medium heat until the liquid has evaporated. Transfer to a bowl and mix in the chopped chives and parsley. Leave covered on a plate to cool to room temperature.
3. Cover the tenderloin with mushroom mixture and then place prosciutto slices, slightly overlapping each other, over the mushrooms and around the tenderloin.
4. Using a rolling pin, roll dough into a slightly bigger rectangle and place on a baking sheet lined with parchment. Place tenderloin in the middle of the pastry. Fold the pastry tightly around the tenderloin. The dough should overlap 1/2 inch. Trim off any excess dough and press the edges together firmly. Turn the meat gently so that the seam is not visible. Cut decorative shapes out of the leftover pastry and press them onto the top of the Beef Wellington.
5. Brush pastry with egg. Bake about 40 minutes, or until an instant read thermometer reaches 135oF.
6. Remove from oven and let rest, loosely covered with foil, for 10 minutes. Cut into thick slices.
Per serving: 397calories, 23g fat, 8g saturated fat, 45mg cholesterol, 446mg sodium, 15g carbohydrate, 1g fiber, 1g sugar, 32g protein