6 tarragon stalks, leaves stripped and then roughly chopped
1 egg, beaten
1 sheet of ready roll puff pastry
Salt and pepper
In a large sautee pan, heat up a little olive oil and then place your chicken pieces in. With the heat fairly high, get some colour on your chicken pieces, turning them over every now and again, before turning the heat down a little and then adding in your onions and mushrooms. Add some seasoning and cook for about ten minutes or so or until the onions are tender.
Sprinkle your cornflour into the pan, and stir it in, so that the chicken and mushroom pieces are coated, then pour in your milk and the mustard, then simmer for 5 minutes or until your sauce begins to thicken. Then take off the heat, stir in your chopped tarragon, then pour into your pie dish and set aside to cool
Preheat your oven to 200 degrees / 180 fan
Once your pie filling is cool, brush around the edge of the pie dish with beaten egg, and then lay your pastry sheet over the top, and cut around the dish, leaving a little over hang so that you can seal the pastry. Then using a fork, press the pastry down onto the rim to seal. Cut a cross in the middle of your pie, then brush the whole top with beaten egg before placing into a hot oven
Cook for 30 minutes or until the pastry is golden and has risen.