Step 1: Cook the chicken
- Heat the olive oil in a large frying pan over medium heat. Add the chicken pieces and cook for about 5-7 minutes until browned on all sides. Remove the chicken from the pan and set aside.
Step 2: Sauté the vegetables
- In the same pan, add the chopped onion, leeks, and garlic. Sauté for 5-7 minutes until the leeks are soft and the onions are translucent. Stir in the thyme, if using.
Step 3: Make the sauce
- Sprinkle the flour over the leek mixture and stir to combine. Gradually pour in the chicken stock, stirring constantly to avoid lumps. Let the sauce simmer for 2-3 minutes until it thickens slightly. Add the double cream and Dijon mustard, stirring until the sauce is smooth and creamy.
Step 4: Combine the chicken and sauce
- Return the cooked chicken to the pan and stir to combine with the sauce. Season with salt and pepper to taste. Remove the pan from heat and let the mixture cool slightly.
Step 5: Assemble the pie
- Preheat your oven to 200°C (400°F). Transfer the chicken and leek mixture into a pie dish. Roll out the puff pastry and lay it over the top of the pie dish, trimming off any excess pastry. Use a fork to press down the edges and seal the pie. Brush the top of the pastry with the beaten egg to give it a golden finish.
Step 6: Bake the pie
- Place the pie in the preheated oven and bake for 20-25 minutes, or until the pastry is golden and puffed. Remove from the oven and let it rest for a few minutes before serving.