Begin by seasoning the boneless, skinless chicken breasts with salt and pepper. Heat a tablespoon of olive oil in a skillet over medium-high heat. Sear the chicken for about 3–4 minutes on each side until golden brown. Remove the chicken from the pan and allow it to cool completely before wrapping it later in the Chicken Wellington.

In the same skillet, melt 2 tablespoons of butter over medium heat. Add finely chopped mushrooms, shallots, and minced garlic, sautéing for about 5–7 minutes until the mixture is golden and moisture has evaporated, creating a paste-like consistency. Set aside the duxelles to cool completely before combining with the chicken.

On a sheet of plastic wrap, layer slices of prosciutto, slightly overlapping them. Spread the cooled mushroom duxelles evenly over the prosciutto, then place the cooled chicken in the center. Using the plastic wrap, tightly roll the chicken and stuffing into a log shape. Chill this roll in the refrigerator for about 15–20 minutes to ensure it holds together well in the Chicken Wellington.

While the chicken roll chills, preheat your oven to 400°F (200°C). On a lightly floured surface, roll out the puff pastry into a rectangle, approximately 1/8 inch thick, ensuring it remains cold for a better rise. Place the chilled chicken roll at the center of the pastry, making it ready for a beautiful envelopment.

Carefully fold the puff pastry over the chicken roll, sealing all edges tightly to prevent any filling from escaping. If desired, use a fork to crimp the edges for a decorative touch. Once wrapped, chill the Chicken Wellington in the refrigerator for 10 minutes to firm up before baking.

In a small bowl, whisk an egg yolk with a splash of water to create an egg wash. Brush this mixture generously over the top and sides of the Chicken Wellington, giving it a beautiful golden finish. Use a sharp knife to cut small slits in the pastry for steam to escape during baking, ensuring a flaky texture.

Place the Chicken Wellington on a baking sheet lined with parchment paper and bake it in the preheated oven for 25–30 minutes, or until the pastry is golden brown and crispy. Keep an eye on it during the last few minutes to achieve your desired level of crispiness.

Once baked, remove the Chicken Wellington from the oven and let it rest for about 5 minutes. This resting time allows the juices to redistribute, ensuring every slice is juicy and flavorful. Slice it carefully and serve to delight your family or guests!