1 x 1.8-2kg whole cooked chicken
600ml good-quality chicken stock
1.25kg floury potatoes (about 5 large ones)
225g cubed pancetta
250g chestnut mushrooms, cut into quarters
800g leeks, cut into fat rounds, 1-2cm thick
3 tbsp roughly chopped fresh tarragon leaves
3 tbsp plain flour
250ml double cream
Salt and freshly ground black pepper
1. Preheat the oven to 200C/fan 180C/gas 6. Wash the potatoes, place on a baking sheet and bake in the oven for about 45 min or until just beginning to soften but not cooked through. Leave until cool enough to handle, then peel away the skins and coarsely grate the flesh into a bowl. Season with salt and pepper. Leave the oven on.
2. Meanwhile, fry the pancetta in a large frying pan over a high heat until just crispy, then remove and set aside. Fry the mushrooms in the fat left in the pan for a couple of minutes, adding a little butter if necessary. Set aside.
3. In a separate pan, melt 50g of the butter and add the leeks and tarragon. Cover with a lid and leave to soften on a low heat for about 10 min. Stir in the flour and cook, stirring, for 2-3 min, then add your 600ml of stock. Bring to the boil, stirring all the time, then reduce the heat and simmer for about 2 min or until the sauce has just thickened slightly. Remove from the heat and stir in the cream. Leave the sauce to cool.
4. Remove the flesh from the chicken and roughly chop, discarding the skin. Stir into the cooled sauce with the pancetta and mushrooms. Taste and season with salt and black pepper as necessary. Spoon into your ovenproof dish.
5. Place mounds of grated potato on top of the chicken mixture, then melt the remaining butter and pour over the top. Stand the dish on a baking sheet and bake in the oven for 40 min or until golden and piping hot.