Put the chicken thighs into a large saucepan with the carrot, celery and white sliced leeks. Pour on the stock, wine, salt and pepper. Bring to the boil, then cover and simmer gently for 1 hour until the chicken is very tender.
Strain the chicken stock into a measuring jug, drain the chicken and vegetables then transfer the chicken to a chopping board and cut the meat into small pieces discarding the skin and bones and vegetables. If the stock measures more than 900 ml (1½ pints) return it to the saucepan and boil rapidly until reduced.
Melt the butter in a smaller saucepan, add the green sliced leeks and fry for 2–3 minutes until softened. Stir in the flour and cook briefly, then gradually mix in the strained stock and bring to the boil, stirring until the sauce has thickened slightly. Stir in the orange rind, mustard and tarragon. Taste and adjust the seasoning if needed. Divide the diced chicken between 6 x 300 ml (½ pint) ovenproof dishes so that the soup three-quarter fills the dishes (any more and they will overflow during baking).
Unroll the pastry, cut 6 circles slightly larger than the tops of the dishes, then 6 long strips about 1 cm (½ inch) wide from the trimmings. Brush the dish rims with a little egg, stick the pastry strips around the rims, then brush these with egg before sticking the pastry lids in place. Flute the edges of the pastry with a small knife then slash the lids lightly. Brush with egg, sprinkle with a little salt and bake in a preheated oven, 200°C (400°F), Gas Mark 6, for 20–25 minutes until golden and the soup is bubbling underneath. Stand the dishes on small plates and serve immediately.