Preheat your oven to 400°F (200°C).

In a large skillet, heat the olive oil and butter over medium heat. Add the sliced leeks and mushrooms, cooking until softened (about 5-7 minutes).

Stir in the shredded chicken, thyme, salt, and pepper. Pour in the chicken broth and heavy cream, stirring until well-combined. Allow this mixture to simmer for about 5 minutes until slightly thickened.

On a lightly floured surface, roll out the puff pastry to fit your pie dish, leaving some overhang for a crust.

Pour the creamy chicken mixture into the pie dish lined with the pastry. Fold the overhang over the filling, sealing it gently. Brush the top with beaten egg.

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

Once done, allow it to cool for a few minutes before slicing.

Serve warm at your next family gathering and enjoy!