1. Heat oil in a medium saucepan over high heat. Cook leek, carrot, garlic and thyme, stirring, for 5 minutes or until vegetables are softened. Add flour; cook, stirring, for 1 minute. Pour in stock and cream; bring to the boil. Reduce heat to low; simmer, stirring occasionally, for 4 minutes or until reduced slightly. Stir in vinegar and chicken until combined. Season to taste.
2. Spray a 4-hole (2-tablespoon) sausage roll maker with oil.
3. Cut each pastry sheet in half. With the machine turned off, position a half pastry sheet so it covers the four holes. Press pastry gently into holes. Fill each hole with ¼ cup chicken filling. Spread 2 tablespoons mashed potato evenly over filling. Spray top with oil.
4. Turn machine on. Close lid; cook for 12 minutes or until golden. Transfer pies to a tray; cover to keep warm. Turn machine off for 5 minutes to cool.
5. Repeat steps 3 and 4 in batches with two more half pastry sheets, more chicken filling and mashed potato.
6. Cut the remaining half pastry sheet crossways in half. Position one pastry piece to cover the two left (front and back) holes (discard remaining pastry piece or save for another use); continue with step 3, using remaining chicken filling and mashed potato. Spray top with oil. Repeat step 4.
7. Use a sharp knife to cut batches into separate pies. Serve pies topped with extra thyme leaves.
You will need 1 whole barbecued chicken for this recipe. If you like, omit the mashed potato and use puff pastry on the top as well as the bottom for a classic chicken hand pie.