Preheat your oven to 400°F (200°C). Pat the chicken breasts dry. Season with salt, pepper, and a thin smear of Dijon mustard if using. In a skillet over medium-high heat, add olive oil and sear the chicken for 2-3 minutes per side until golden but not fully cooked. Remove and let cool completely.

Finely chop the mushrooms and shallots (or onion) until they resemble coarse sand; mince the garlic. In the same skillet, cook the mushroom mixture, garlic, and thyme over medium heat for 10-15 minutes, stirring frequently, until all liquid evaporates and the mixture is dry and concentrated. Season with salt and pepper. Let cool completely.

On a lightly floured surface, unfold the puff pastry and cut into 4 equal squares. Spread a thin layer of cream cheese on each square, leaving a 1-inch border. Place a cooked chicken breast in the center, top with duxelles. Brush edges with egg wash, then fold corners over to seal like a parcel. Transfer to a parchment-lined baking sheet.

Brush the tops with egg wash. Bake for 20-25 minutes until pastry is golden brown and puffed. Let rest 5-10 minutes before serving. Ensure internal temperature reaches 165°F (74°C).