Begin by seasoning the boneless, skinless chicken breasts with salt and black pepper. Heat a tablespoon of olive oil in a large skillet over medium-high heat. Sear the chicken for about 5-6 minutes, turning until golden brown on all sides. Once cooked through, remove the chicken from the skillet and let it cool completely on a plate.

In the same skillet, add two tablespoons of butter and reduce the heat to medium. Sauté finely chopped mushrooms, shallots, and garlic for 8-10 minutes, stirring frequently until the moisture has evaporated and the mixture resembles a paste. Remove the duxelles from heat and allow it to cool to room temperature, enhancing the flavor profile.

Lay slices of prosciutto flat on a sheet of plastic wrap, slightly overlapping. Spread the cooled mushroom duxelles evenly on top of the prosciutto. Place the cooled chicken in the center and roll it tightly, using the wrap to help. Secure the ends tightly and refrigerate the chicken roll for 15-20 minutes to firm up.

Preheat your oven to 400°F (200°C). On a lightly floured surface, roll out the puff pastry into a rectangle large enough to enclose the chicken. Unwrap the chilled chicken roll and place it in the center of the pastry. Fold the pastry over, sealing the edges firmly to prevent leaks, and chill for an additional 10 minutes to firm up.

Beat one egg yolk with a splash of water in a small bowl, then brush the mixture over the wrapped pastry for a glossy finish. Use a sharp knife to make a few small slits on top for steam to escape. Place the Chicken Wellington on a baking sheet and bake in the preheated oven for 25-30 minutes, until the pastry is golden brown and puffed. Allow it to rest for 5 minutes before slicing to retain its juicy filling.