Mix the flour, suet and salt in a large bowl. Add the cold water and work by hand until the pastry dough comes together.
Do not overwork. If it becomes sticky you have overworked it.
If the pastry seems dry, add a splash more water to bring it together. Wrap in cling film and chill for 30 minutes.
Preheat oven to 200'C.
Shred the chicken breasts. Slice the mushrooms. Peel and dice the shallot and the garlic. Pick the leaves from the tarragon and roughly chop.
Heat a frying pan until hot and add a splash of sunflower oil. Cook the mushrooms until golden brown and then season with salt and pepper.
Heat a saucepan pan over a medium heat and cook the shallots in the butter until golden brown. Add the garlic and cook for 1 minute. Next, add the flour and cook for 1 minute and then add the hot chicken stock and bring to the boil.
Reduce the heat and cook for 3 minutes. The sauce will thicken as the flour in the sauce cooks. Remove from the heat and stir in the tarragon, shredded chicken and sauteed mushrooms.
Check the seasoning. Put the filling in the pie tin and allow to cool.
Whilst the filling is cooling, roll out the pastry to 5 mm thick and so as it is at least 3 cm wider and longer than the pie tin.
Top the tin with the pastry and crimp the edges. Brush the pie lid with a little beaten egg and then make a small hole in the centre of the pie lid.
Bake in the oven for 30 minutes or until the pastry is golden brown.
«The hole in the centre of the pie will allow the steam from the filling to evaporate as it cooks. This will stop the pie crust ending up soggy.»