Creating Puff Pastry Chicken and Leek Casserole is simple when you follow these steps:

1. Preheat Oven: Start by preheating your oven to 400°F (200°C).
2. Sauté Chicken: In a large skillet, heat olive oil over medium heat. Add diced chicken, season with salt and pepper, and cook until golden brown, about 5-7 minutes.
3. Add Vegetables: Add leeks and garlic to the skillet. Sauté for an additional 5 minutes, until leeks are soft.
4. Make Sauce: Sprinkle flour over the chicken and leek mixture, stirring to coat. Gradually pour in the chicken broth and heavy cream, stirring constantly until the mixture thickens.
5. Season: Stir in dried thyme and adjust seasoning with more salt and pepper if necessary. Remove from heat.
6. Prepare Baking Dish: Pour the chicken and leek mixture into a greased baking dish, spreading it evenly.
7. Roll Out Puff Pastry: On a lightly floured surface, roll out puff pastry to fit the size of your baking dish.
8. Cover Casserole: Place the rolled pastry over the casserole, pressing the edges to seal. Cut a few slits in the pastry to allow steam to escape.
9. Egg Wash: Brush the top with the beaten egg for a beautiful golden finish.
10. Bake: Place the baking dish in the preheated oven and bake for 25-30 minutes, or until the pastry is golden and flaky.
11. Rest: Remove from the oven and let it sit for 10-15 minutes before serving.

Following these instructions will guide you to create a stunning and hearty casserole that delights the senses.