Step 1

Bring stock to the boil in a medium saucepan over high heat. Add chicken; return to the boil. Reduce heat and simmer, covered, about 10 minutes or until chicken is cooked. Remove chicken and chop coarsely. Reserve 1 cup of the poaching liquid.

Step 2

In a medium saucepan, heat butter; cook leek and celery, stirring, until leek softens. Add flour and thyme; cook, stirring, 1 minute. Gradually stir in reserved poaching liquid, milk and cream; cook, stirring, until mixture boils and thickens. Stir in chicken and mustard; season. Cool.

Step 3

Preheat oven to 220°C (200°C fan-forced). Grease a 1-litre (4-cup) pie dish with rim. From one sheet pastry, cut 4 strips slightly wider than dish rim; arrange strips on rim, pressing firmly until rim is covered. Brush with a little egg yolk.

Step 4

Spoon filling into dish; roll out remaining pastry sheet slightly to fit pie dish. Place over filling. Press edges to seal; trim. Brush pastry with egg yolk; cut two small slits in top. Bake 25 minutes or until puffed and browned. Sprinkle with extra thyme.