1. Grease 4 pie tins 12cm x 6cm deep.
2. Line tins with pastry and place in fridge.
3. Heat oil in a large saucepan. Add chicken, chicken stock powder and tarragon. Cook for 2 minutes until slightly golden in colour. Remove from pan and reserve.
4. Add butter to same saucepan. When melted, add leeks and cook for 2 minutes. Add mushrooms and celery, stir and cook for a few more minutes.
5. Once the mushrooms have softened, add bok choy, chicken stock and wine, then stir well. Add browned chicken and crème fraiche. Mix well, then cool.
6. Preheat the oven to 180 degC. Divide mixture into prepared chilled pastry cases.
7. Cover and seal pies with remaining pastry, then brush with egg wash. Bake for 40 to 45 minutes.

Cook's tip:Place any defrosted leftover pastry well-covered in the fridge and use within the next two days. These pies can be made the day before. Brush with egg wash when oven is heating.