Heat the olive oil in a large pan over medium heat. Add the chicken pieces and cook until browned all over. Remove from the pan and set aside.

In the same pan, add the butter and allow it to melt. Toss in the leeks, garlic, and diced bacon (if using). Cook until the leeks are softened and the bacon is crisp.

Sprinkle the flour over the leek mixture, stirring well to combine. Gradually add the chicken stock, stirring continuously to avoid lumps. Let the mixture simmer for a few minutes until it thickens.

Stir in the double cream, Dijon mustard, and thyme. Season with salt and pepper to taste. Add the cooked chicken back into the pan, mixing everything well. Let the filling cool slightly.

Preheat your oven to 200°C (180°C fan) or 400°F.

If using ready-rolled puff pastry, roll it out slightly on a floured surface to fit your pie dish. If using shortcrust pastry, ensure it’s rolled out evenly.

Spoon the chicken and leek filling into your pie dish. Cover with the pastry, pressing the edges down to seal. Trim any excess pastry.

Use a fork to crimp the edges of the pastry. If you like, you can use the pastry trimmings to create decorative shapes on top. Brush the entire pastry with beaten egg for a golden finish.

Place the pie in the preheated oven and bake for 25-30 minutes, or until the pastry is golden brown and puffed up.

Remove from the oven and let it sit for a few minutes before serving.