Creating Chicken Wellington can be straightforward if you follow these detailed steps:

1. Preheat Oven: Preheat your oven to 400°F (200°C) and line a baking sheet with parchment paper.
2. Cook Mushrooms: In a skillet, heat olive oil over medium heat. Add the chopped mushrooms and sauté until they release their moisture and become golden brown. Season with salt, pepper, and thyme.
3. Prepare Chicken: Season the chicken breasts with salt and pepper. Sear them in the same skillet for about 2-3 minutes on each side until golden brown. Remove from heat and let cool slightly.
4. Spread Mustard: Once cooled, spread a thin layer of Dijon mustard over each chicken breast.
5. Assemble Layers: On a piece of plastic wrap, spread the sautéed mushrooms evenly. If using, add a layer of pâté on top of the mushrooms.
6. Wrap Chicken: Place the seasoned chicken on the mushroom layer and roll the plastic wrap tightly, creating a log shape. Chill in the refrigerator for 15 minutes.
7. Prepare Pastry: Roll out the puff pastry on a lightly floured surface to ensure it’s large enough to encase the chicken.
8. Unwrap Chicken: Remove the chicken from the plastic wrap and place it in the center of the puff pastry.
9. Seal the Pastry: Fold the pastry over the chicken, trimming any excess. Pinch the edges to seal and place seam-side down on the baking sheet.
10. Apply Egg Wash: Brush the entire pastry with the beaten egg for a golden finish.
11. Bake: Place in the preheated oven and bake for 40-45 minutes or until the pastry is golden and crisp.
12. Cool Before Slicing: Remove from the oven and allow to cool for about 10 minutes before slicing.

These steps will guide you in creating a stunning Chicken Wellington that is as delicious as it is beautiful.