Method
Preheat the oven to 180°C/Gas 4. Boil all the potatoes in a large saucepan of lightly salted water for 15-20 minutes or until tender. Drain and return to the pan with the stock, mash until smooth and season well. Stir in the parsley and set aside.
To make the filling place the stock, leeks, garlic, carrots, peas and mushrooms in a saucepan and cook over a gentle heat for 12-15 minutes, stirring often, until the leeks have softened. Add the chicken to the pan and stir to mix well.
Add the tarragon, season well and cook gently for 3-4 minutes, stirring often. Remove from the heat and transfer to a medium-sized pie dish.
Spread the mash over the chicken mixture and brush the top with the beaten egg. Bake in the oven for 20-25 minutes or until golden and bubbling. Serve immediately with steamed green beans.