1.Bring stock to the boil in small saucepan. Add chicken, return to the boil. Reduce heat, simmer, covered, about 10 minutes or until chicken is cooked through.
2.Remove from heat, stand chicken in poaching liquid 10 minutes. Remove chicken chop finely. Reserve 1/4 cup of the poaching liquid, discard remainder.
3.Heat oil in medium saucepan, cook leek and celery, stirring, until leek softens. Add flour and half the thyme, cook, stirring, 1 minute. Gradually stir in reserved liquid and cream, cook, stirring, until mixture boils and thickens. Stir in chicken and mustard, season to taste. Cool 10 minutes.
4.Preheat oven to 220°C. Oil eight holes in each of two 12-hole (2-tablespoon/40ml) deep flat-based patty pans.
5.Cut 16 x 7cm rounds from shortcrust pastry, press one round into each of the prepared holes. Spoon 1 tablespoon chicken mixture into each pastry case. Cut 16 x 6cm rounds from puff pastry, top chicken pies with puff pastry lids.
6.Brush lids with egg yolk, sprinkle with sesame seeds and remaining thyme. Using sharp knife, make two small slits in each lid. Bake, uncovered, about 20 minutes or until browned lightly.
Chicken mixture can be made the day before and kept, covered, in the refrigerator.