Heat oven to 200°C / 180°C (fan oven) / 395°F / Gas Mark 6. In a roasting tray toss the sweet potatoes with 2 tsp of the oil and some seasoning. Cook for 30-40 mins, until golden and crisp. Heat 1 tsp oil in a medium frying pan. Fry the chicken until browned, remove from the pan and set aside. Add the leek and a splash of water, and gently fry until soft, about 7 mins. Add the carrot and cook for 3 mins more. Pour in the stock and boil until reduced by half, then add the mustard and soft cheese, stirring well to combine. Return the chicken to the pan, add the tarragon and some seasoning.

Divide the mixture between 2 small ovenproof dishes. Take the filo sheets and scrunch them up. Top each pie with a sheet and brush with remaining 1 tsp oil. Cook the pies in the oven with the chips for 15 mins, until the pastry is golden.