Preheat oven to 220°C. Place chicken in a deep pot and add bay leaves, onion and peppercorns. Cover chicken with water, about 3–4 cups. Poach chicken for half an hour, or until cooked through. Remove from pot, strain liquid and set aside.
Heat butter, add leeks and cook until translucent. Add flour and stir for 1–2 minutes. Whisk in the remaining chicken liquid, add wine and cook until mixture thickens.
Shred cooled chicken, discarding the skin. Add shredded chicken to leek mixture, stir in sour cream, tarragon and potatoes. Season. Transfer to a baking dish of about 22cm.
Roll out the pastry on a floured surface and cut to the desired shape. Moisten the edges of the dish with egg white and place pastry over the dish. Trim edges and secure with the tines of a fork.
Make a small slit in the centre of the pastry. Decorate with leftover pastry and brush with the egg wash. Bake for 20–30 minutes or until puffed up and golden.
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