- 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!