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

In a large sauté pan, heat olive oil over medium heat. Add chopped mushrooms and cook for 5–7 minutes until browned. Stir in spinach, seasoning with salt and pepper, then remove from heat and allow to cool.

Season boneless chicken breasts on both sides with salt, pepper, and Dijon mustard. Let it sit while preparing for assembly.

Roll out the chilled puff pastry on a floured surface. Spoon the cooled mushroom and spinach filling into the center, place the seasoned chicken on top, then fold the pastry edges to seal tightly.

Brush the top of the assembled Wellington with beaten egg, and bake for 30–35 minutes until golden and the internal temperature of the chicken reaches 165°F (74°C).

Simmer heavy cream and chicken broth in a medium saucepan over medium heat, then add Dijon mustard and let the sauce thicken for about 5 minutes.