Making a Chicken and Leek Pie can be straightforward if you follow these simple steps:

1. Preheat the Oven: Begin by preheating your oven to 400°F (200°C).
2. Sauté the Onions and Leeks: In a large skillet, heat olive oil over medium heat. Add the chopped onions and sliced leeks. Sauté until they are soft and translucent, about 5-7 minutes.
3. Add Garlic: Stir in the minced garlic and cook for an additional minute, being careful not to burn it.
4. Cook the Chicken: Add the diced chicken to the skillet, cooking until it’s browned on all sides, around 5-8 minutes.
5. Make the Sauce: Pour in the chicken broth and heavy cream. Add the Dijon mustard, thyme, and parsley. Stir well to combine.
6. Season the Mixture: Season with salt and pepper to taste. If you prefer a thicker sauce, dissolve cornstarch in a small amount of cold water and mix until smooth, then add to the skillet.
7. Simmer the Filling: Allow the mixture to simmer for about 10-15 minutes, stirring occasionally until the chicken is cooked through, and the sauce thickens.
8. Prepare the Pie Dish: Transfer the chicken and leek filling to a pie dish, spreading it evenly.
9. Roll Out the Pastry: Roll out the puff pastry on a lightly floured surface. Place it over the pie dish, pressing the edges to seal. Trim any excess pastry as needed.
10. Apply the Egg Wash: Brush the top of the pastry with the beaten egg to achieve a golden finish.
11. Cut Air Vents: Use a sharp knife to cut a few slits in the top crust to allow steam to escape while baking.
12. Bake the Pie: Place in the preheated oven and bake for 45 minutes or until the pastry is golden and crisp.
13. Cool Before Serving: Once baked, allow the pie to cool for 10 minutes before slicing and serving.

These steps will guide you in creating this incredible Chicken and Leek Pie with ease!