- Preheat the oven to 200°C/gas 6. Heat the oil in a saucepan add the chicken and leeks and fry for 4-5 minutes until the chicken begins to brown.
- Stir in the flour and 450ml of the milk, bring to the boil and stir until thickened. Stir in the tarragon half the cheese and the butter beans and season well. Tip into an ovenproof dish.
- Mix together the mashed potato, remaining milk, butter and the remaining cheese and pipe or spoon over the chicken mixture.
- Place the pie in the oven and cook for 20-25 minutes until golden and bubbling.
- Try using sweet potato rather than ordinary potato as it counts as one of your 5 a day.
- Freezing instructions: Suitable for freezing once cooked. Defrost in the fridge and reheat until piping hot throughout.