Heat a frying pan over medium heat. Add a little oil and cook the diced chicken. Stir often until fully cooked. The chicken should turn white and slightly golden. Remove and set aside.

In the same pan, add butter and sliced leeks. Cook gently for about 5 minutes. Stir often so they do not burn. The leeks should become soft and slightly sweet.

Add flour to the leeks and mix well. Cook for one minute. Slowly pour in chicken stock while stirring. Then add milk. Keep stirring until the sauce becomes smooth and thick.

Return the cooked chicken to the pan. Mix well with the sauce. Add salt, pepper, and thyme. Let it simmer for a few minutes. The filling should be creamy and rich.

Set your oven to 180°C. Let it heat properly. A hot oven helps the pastry cook evenly and turn golden.

Pour the chicken and leek mixture into a pie dish. Spread it evenly. Let it cool slightly before adding pastry on top.

Roll out the puff pastry if needed. Place it over the pie dish. Trim extra edges. Press the sides gently to seal the pie.

Use a brush to spread beaten egg over the pastry. This gives a golden color. Make a small cut in the center to let steam escape.

Place the pie in the oven. Bake for about 30–35 minutes. The top should become golden and crispy. The filling inside should be hot and bubbling.

Remove from the oven carefully. Let it cool for a few minutes. Serve warm. Enjoy the creamy filling and flaky crust.