- Preheat the oven to 180°C/ gas 4. Add the oil to a pan, then add the onion and stir regularly for 5 minutes until browned.
- Add the leeks and mushrooms, and stir regularly for 3–4 minutes, until softened.
- Stir in the stock, tarragon and pepper, and bring to the boil. Add a lid to the pan, reduce heat and simmer for 5 minutes.
- Mix the cornflour and milk together. Next, bring the stock back to the boil and slowly stir in the cornflour mixture until the sauce thickens, then stir for another 2 minutes.
- Now, stir in the cooked chicken pieces, then add mixture to an overproof dish.
- Evenly layer the sweet potato on top and bake in the oven for 25–40 minutes until lightly browned.