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

Season chicken breasts with salt and pepper. Heat skillet with olive oil and sear chicken for 4-5 minutes per side until golden.

In the same skillet, add chopped mushrooms and cook for 5 minutes, then add spinach and sauté until wilted.

Remove skillet from heat, chop chicken, and combine with sautéed vegetables and Dijon mustard in a bowl.

Roll out puff pastry to 1/8-inch thickness, cut into rectangles large enough to enclose the filling.

Place filling in the center of each pastry rectangle, fold and seal edges tightly.

Brush the tops with egg wash for a golden finish before placing them in the baking dish.

Bake in preheated oven for 25-30 minutes until golden brown and puffed.

While baking, prepare the Dijon sauce by cooking heavy cream, chicken broth, garlic powder, and Dijon mustard together for 5-7 minutes.

Slice each Chicken Wellington in half and serve with the creamy Dijon sauce drizzled on top.