1. Set the oven to fairly hot Gas Mark 6 or 200Â°C/400Â°F.
2. To make the filling, pour the soup and water into a saucepan and add the onion, celery and carrots. Bring to the boil, then reduce the heat and simmer gently for 10 minutes. Stir in the turkey and simmer for a further 10-15 minutes, until the turkey has just cooked through. Remove the pan from the heat and stir in the green beans. Pour into a casserole dish.
3. To prepare the topping, place the potatoes in a large saucepan, cover with water and bring to the boil. Reduce heat and simmer for 5 minutes, then remove the pan from the heat and leave until the potatoes are just cool enough to handle.
4. Scrape the skins from the potatoes and then grate coarsely. Mix with salt and pepper and then spread over the turkey filling. Spray over a little oil.
5. Place the dish on a baking tray and bake in the centre of the oven for 30-35 minutes,or until the topping is golden. Remove from the oven and serve immediately.
For the filling:
For the topping: