1.Combine chicken and stock in small saucepan; simmer, covered, about 10 minutes or until chicken is cooked through. Remove from heat; cool chicken in stock to room temperature. Remove chicken; chop finely. Reserve ¾ cup of the stock.
2.Melt butter in same saucepan, add leek and celery; cook, stirring, until soft. Stir in flour and mustard. Gradually stir in reserved stock; stir over heat until mixture boils and thickens. Add chicken and parsley. Cool.
3.Preheat oven to 220°C (200°C fan-forced). Oil two 12-hole (⅓-cup/80ml) muffin pans.
4.Cut 24 x 7cm rounds from pastry sheets; press rounds into each pan hole. Divide chicken mixture among pastry cases; brush cases with egg. Cut 24 x 6cm rounds from remaining pastry; position on top of pastry cases, press edges with fork to seal. Cut a small cross in top of each lid. Bake about 25 minutes or until browned.
5.Stand 10 minutes before serving with tomato sauce.
Pies can be cooked one day ahead; keep, covered, in refrigerator. Reheat pies in a single layer on oven trays, covered loosely with foil, in oven (180°C/160°C fan-forced) for about 10 minutes. Uncooked pies can be frozen between layers of freezer wrap for up to three months; thaw in refrigerator for 12 hours or overnight before cooking as per recipe