Preheat your oven to 400°F (200°C). Line a baking dish with parchment paper or lightly grease it.
In a large pan, melt 3 tablespoons salted butter over medium heat. Stir in 3 tablespoons all-purpose flour to create a roux, cooking for about 1-2 minutes. Gradually whisk in the 1 cup milk and 1 cup heavy cream, and cook until the mixture thickens, about 5 minutes.
Add 1 teaspoon salt, 1 teaspoon black pepper. 1 teaspoon onion powder, ½ teaspoon garlic powder, ¼ teaspoon celery seed, 1 teaspoon poultry seasoning and 1 teaspoon granulated white sugar Stir well to combine.
Stir in the cooked 2 cups chicken, 12 ounces vegetables and boiled 1 large potato. Mix until everything is evenly distributed. Remove from heat.
Roll out the thawed 17.3 oz puff pastry on a lightly floured surface. Cut it to fit your baking dish, ensuring it will cover the filling.
Pour the chicken mixture into the prepared dish. Place the puff pastry over the filling, pressing the edges to seal. Use a knife to create small slits on top for steam to escape.
In a small bowl, whisk together the 1 large egg and w1 teaspoon water to create an egg wash. Brush the top of the puff pastry with this mixture for a golden, crispy crust.
Place the pie in the oven and bake for 30-35 minutes, or until the puff pastry is golden brown and crispy.
Let the pot pie cool for a few minutes before slicing and serving. Enjoy your homemade, KFC-inspired chicken pot pie!