Heat the butter with a splash of olive oil in a large frying pan over medium heat.

1. Add the chicken, leeks, fennel seeds and tarragon. Cook until the chicken is around three-quarters cooked through.

2. Remove the mixture from the pan and set aside.

3. In a saucepan, melt the butter and stir in the flour to create a roux. Cook for around 5 minutes.

4. Gradually whisk in the warm milk followed by the chicken stock until smooth and thick enough to coat the back of a spoon.

5. Stir through the Dijon mustard, mozzarella and the cooked chicken mixture.

6. Simmer gently for 10–15 minutes before allowing the filling to cool completely.

7. Line a pie dish with puff pastry, add the cooled filling and top with another pastry sheet.

8. Seal the edges, brush with beaten egg and sprinkle with extra fennel seeds and tarragon.

9. Bake at 180°C for 20–25 minutes, or until the pastry is golden and crisp.