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

In a skillet over medium heat, sauté finely chopped mushrooms in olive oil until golden brown, about 5-7 minutes. Add spinach and sauté until wilted, approximately 2 minutes. Season with salt and pepper, then cool.

Season both sides of the boneless chicken breasts with salt and pepper. Spread a thin layer of Dijon mustard on each breast.

Roll out the thawed puff pastry on a floured surface. Place the cooled spinach and mushroom mixture in the center, top with chicken, and fold the pastry over the filling, sealing edges.

Transfer the assembled Wellington to the baking sheet, brush with egg wash, and bake for 30-35 minutes until golden brown and chicken reaches 165°F (75°C).

For the sauce, combine heavy cream and chicken broth in a saucepan over medium heat. Stir in Dijon mustard and simmer until thickened, about 5-7 minutes.

Once baked, let the Chicken Wellington rest for 5 minutes before slicing and serving with the Dijon cream sauce.