Preheat the oven to 400°F (200°C).

Trim any excess fat from chicken breasts. Use a meat mallet or heavy pan to gently pound each breast to an even thickness of about 3/4 inch. Season both sides with a pinch of salt and a few twists of black pepper. Spread about 1/2 teaspoon Dijon mustard on the top of each breast.

Heat olive oil in a skillet over medium heat. Add the chopped mushrooms and a pinch of salt. Sauté until the mushrooms release their liquid and the moisture evaporates, about 5-7 minutes. Stir occasionally to ensure even browning. Season with black pepper and fold in half of the chopped fresh herbs. Remove from heat and let cool slightly.

Lay one slice of smoked turkey on each seasoned chicken breast. Spoon the sautéed mushrooms evenly over the turkey layer. Roll the breast up tightly around the filling, tucking in ends as you go. Secure each roll with a toothpick or kitchen twine if needed.

Lightly flour your work surface and briefly roll the puff pastry to smooth it out. Place a rolled chicken seam-side down in the center of the pastry. Fold both sides of the pastry over the chicken, sealing the seam beneath the roll. Trim any excess pastry and tuck the ends under.

Transfer the wrapped chickens onto a lined baking sheet, seam-side down. Brush the tops evenly with the beaten egg. If desired, score a few shallow decorative lines on top.

Bake in the preheated oven for 25-30 minutes or until the pastry is deep golden and a thermometer inserted into the thickest part of the chicken reads 165°F (74°C). Tent loosely with foil if the pastry browns too quickly.

Remove from the oven and let the Wellington rest on the baking sheet for 5 minutes before slicing. Remove any toothpicks or twine and slice on a diagonal to reveal the layered interior.