To make the pastry, place flour, butter and salt into the Thermomix bowl and mix 10 sec/speed 6, or until mixture resembles breadcrumbs. Add egg yolks and water and knead 30 sec/Interval. Knead pastry into a disc shape and wrap up in a floured silpat mat and refrigerate for at least 30 minutes.
To make the Pie filling place garlic, onion, leek and EVOO into TM bowl and chop 3 sec/speed 6. Cook 5 min/Varoma/speed 1.
Add potatoes and tomato and chop 10 sec/speed 8. Add stock and cook 20 min/100/speed 1. Add sour cream, mustard and cornflour. Cook 4 min/90ºC/speed 4. Add peas and stir to combine. Pour filling into a large bowl.
Place chicken onto Varoma tray and sprinkle with tarragon and place carrot into the Varoma dish. Place 1 litre of water into the Thermomix bowl and set Varoma into position. Cook 25 min/Varoma/speed 3. Let cool slightly then add to the filling. Taste and season with salt and pepper.
To assemble Pie, Roll out 2/3 of the pastry on a floured bench to fit the base and sides of your pie dish, ensuring there is a 2cm overhang around the edge.
Place filling into the pastry base, then roll out the top piece of pastry, ensuring you have an overhang of at least 2.5cm. Dab a mix of water and flour around the edge of the pastry base (to act like glue to keep the two pieces together) and place pastry top on the pie. Press two edges together, then cut off excess pastry leaving 2cm hanging over the side. Fold the overhang of pastry neatly under the rim of the pie dish or if using a flan dish, tuck the pastry back into the top inside of the dish pressing against it to ensure pie is fully sealed. Use the leftover pastry to decorate pie.
Brush with egg wash. Cut some slits into the top of the pie to allow steam to escape.
Turn oven on to 200ºC (fan forced), and put the pie into the fridge to allow the “glue” to set and the oven to heat. Bake pie for 15 minutes, turn pie around to allow for even browning, and reduce temp to 180ºC. Continue to bake for a further 20-25 minutes until golden brown.
The pastry and filling can be prepared a day ahead, and then put together earlier the next day and left in the fridge until ready to bake for dinner.