Preheat your oven to 400°F (200°C) and prepare a baking dish with nonstick spray.

In a skillet over medium heat, add olive oil and sauté finely chopped mushrooms for 5-7 minutes until golden brown. Add spinach and cook until wilted, about 2-3 minutes.

Season chicken breasts with salt and pepper, then coat both sides with Dijon mustard. Combine the chicken with the sautéed mushroom and spinach mixture.

Roll out puff pastry on a floured surface until 1/8 inch thick. Place chicken and vegetable mixture in center, fold pastry over filling and seal edges tightly.

Place the wrapped Chicken Wellington seam-side down in the baking dish, brush with egg wash, and bake for 25-30 minutes until puffed and browned.

While baking, make the Dijon sauce by combining heavy cream, chicken broth, garlic powder, and additional Dijon mustard over medium heat for 5-7 minutes until thickened.

Once baked, let the Chicken Wellington rest, then slice and drizzle warm Dijon sauce over the top before serving.