Preheat your oven to 400°F (200°C) and prepare a baking sheet lined with parchment paper.

Melt a tablespoon of butter and a drizzle of olive oil in a skillet over medium heat. Add mushrooms, garlic, and shallots, cooking until thickened, about 10-12 minutes. Season with thyme, parsley, salt, and pepper. Transfer to a bowl to cool.

Season chicken breasts with salt and pepper, and sear in a skillet over high heat for about 4-5 minutes on each side until golden brown. Let cool.

Roll out the puff pastry on a lightly floured surface to about 1/8 inch thick and lay it on the prepared baking sheet.

Spread cooled mushroom mixture over the center of the pastry, leaving an inch border. Place seared chicken on top, then spread cream cheese or Boursin over the chicken.

Fold pastry over the filling, sealing it tightly. Score the top for steam release and decorative effect.

Whisk egg yolk with a tablespoon of water and brush it over the pastry.

Chill the wrapped Wellington for 15-20 minutes before baking.

Bake in the preheated oven for 25-30 minutes until golden brown and chicken reaches 165°F (75°C).

Let chicken Wellington rest for 5-10 minutes before serving.