Preheat the oven to 180°C/Gas 4.
Boil all the potatoes for 15-20 minutes or until tender then mash.
To make the filling place the stock, leeks, mushrooms and carrots in a saucepan and cook over a gentle heat for 10 minutes, stirring often until soft.
Add the peas, tarragon and diced chicken to the filling mix and cook gently for 5 minutes.
Transfer the filling to a casserole or pie dish and top with mash.
Brush the top with all the beaten egg.
Bake in the oven for 20-25 minutes until golden and bubbling.
Serve immediately with extra veg of your choice. I used cauliflower, broccoli, kale and chantenay carrots.