Season both sides of the chicken breasts generously with salt and pepper. Set aside at room temperature for about 10 minutes while you prepare the filling.

Heat a skillet over medium heat with a light drizzle of oil. Add the finely chopped mushrooms and cook for 6 to 8 minutes, stirring regularly, until golden brown and completely dry. All moisture must be cooked off to prevent a soggy pastry.

Add the fresh spinach and stir until fully wilted, about 1 to 2 minutes. Stir in 2 tablespoons of Dijon mustard and mix well. Remove from heat and let the filling cool for at least 5 minutes before assembling.

Preheat oven to 400 degrees F (200 degrees C) and lightly grease a baking dish. On a lightly floured surface, roll out the thawed puff pastry and cut into sections large enough to fully wrap one chicken breast each.

Place one chicken breast near the center of a pastry section and spoon a generous amount of filling on top. Wrap the pastry snugly around the chicken, pinching and folding the seams firmly to seal. Place seam side down in the baking dish.

Brush each Wellington with the beaten egg wash. Bake for 25 to 30 minutes until the pastry is deep golden brown and the internal temperature of the chicken reaches 165 degrees F.

While the Wellingtons bake, combine the heavy cream, chicken broth, garlic powder, and Dijon mustard in a small saucepan over medium heat. Stir frequently for 8 to 10 minutes until the sauce thickens enough to coat the back of a spoon.

Let the Wellingtons rest for 5 minutes after removing from the oven. Slice and serve drizzled generously with the warm Dijon cream sauce.