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

Season the chicken breasts with salt and pepper, and sear them in a skillet over medium-high heat for 4-5 minutes on each side.

In the same skillet, sauté chopped mushrooms for about 5 minutes. Add spinach and cook until wilted, then stir in Dijon mustard.

Roll out puff pastry on a floured surface, place the filling and chicken in the center, wrap the pastry around, sealing edges.

Place the Wellington in the baking dish, brush with egg wash, and bake for 25-30 minutes until golden brown.

While baking, prepare the sauce by combining heavy cream, chicken broth, garlic powder, and Dijon mustard in a saucepan over medium heat.

Once baked, let the Wellington rest, slice, and serve drizzled with the creamy sauce.