Combine stock and wine in a medium saucepan; bring to the boil. Add the chicken, return to the boil. Cover with a lid. Remove from heat; stand chicken for 10 minutes or until it is cooked through. Reserve 3/4 cup (180ml) cooking liquid; chop chicken finely.
Melt butter in a pan; cook leek and celery, stirring, until soft. Stir in flour and thyme and stir until bubbling. Gradually stir in reserved liquid and cream; cook, stirring, until mixture boils and thickens. Stir in the chicken and mustard. Season to taste with salt and black pepper. Remove from heat and cool slightly.
Preheat the oven to hot, 220C/200C fan-forced. Grease 3 x 12-hole patty pans. Using a 7cm cutter, cut 36 rounds from the shortcrust pastry. Press into prepared pans. Spoon 1 tablespoon of chicken mixture into each pastry case. Using a 6cm cutter, cut 36 rounds from the puff pastry. Top chicken mixture with the pastry lids, brush with egg yolk and sprinkle with extra thyme leaves.
Bake on the lowest shelf in a hot oven for 15 minutes or until browned.
NOTE: This recipe makes 36 pies and can be made a day ahead. Suitable to freeze. Filling suitable to microwave.Note