Pre-heat the oven to 180°C.
Add around 3 tbsp of olive to a large frying pan under a medium heat.Cut the chicken breasts up into rough medium sized chunks and add to a bowl with 2 tbsp of flour, salt and pepper. Toss the chicken so it's coated in the flour then add to the pan (dusting off any excess flour). Brown the chicken on all sides for a couple minutes then remove onto a plate, set aside.
Add another tbsp of olive oil to the same pan. Finley chop the onion and add to the pan. Finely slice the mushrooms and add those to the pan after a minute. Lightly fry the mushrooms and onions for 2 minutes then add the chopped garlic. Stir everything together for another minute.
Add the white wine to the pan and let reduce by half then add the stock cube and water. Add the chicken back to the pan and stir occasionally until the sauce is starting to thicken.
Once the sauce had started to thicken add the mascarpone and tarragon with a sprinkling of pepper. Stir together for 1-2 minutes.
If using individual pie dished use these as a guide to cut out the pastry tops. Make sure to leave 1cm extra around the rim. Alternatively just use a normal baking dish and trim the pastry form the eges. Cut out any decorative shapes at this point too.
Divide the chicken and mushroom mixture between the pie dishes on into a baking dish. Brush the edges of the pastry with a little beating egg then top the dishes with the pastry. Use a fork to seal the pastry. Add the pastry trees/shapes then brush the tops with more beaten egg. Make a few small pricks in the pastry for steam to escape with a shape knife.
Bake in the oven until the pastry is puffed and golden (around 25 minuteServe with mashed potatoes and green beans.