Step 1

For the shortcrust pastry, place the flour into a bowl and add the butter. Using the tips of your fingers, gently rub into the flour until it resembles fine breadcrumbs.

Step 2

Stir in 2-3 tbsp cold water using a round bladed knife. When the mixture starts to bind and form a dough, bring it together using your hands. Add 1-2 tbsp more water if needed. Gently and briefly knead with lightly floured hands, to shape and form a flat disc. Wrap and chill for at least 30 minutes before use.

Step 3

Meanwhile, make the filling. Heat the stock in a large saucepan. Cut the chicken thighs into chunks of about 2cm and add to the hot stock. Bring to a gentle simmer, then allow to bubble for 10-15 minutes until the chicken is cooked through with no pink meat and juices that run clear. Lift onto a plate and drain using a slotted spoon. Reserve the stock.

Step 4

Boil and reduce the stock down to 300ml, then pour into a jug. Return the pan to the hob, melt 10g butter, then add the leeks and mushrooms to soften and colour over a medium heat for 5-7 minutes. Transfer to the plate with the chicken and set aside.

Step 5

Melt the remaining 50g butter in the pan, then stir in the flour and cook for 1 minute or so. Remove from the heat while you slowly stir in the 300ml reserved stock. Return to the heat and stir continuously until the sauce is smooth and thick. Season and stir in the cream, tarragon and mustard, then add the cooked chicken, leek and mushrooms and mix well. Leave to cool completely.

Step 6

Preheat the oven to 200ºC, gas mark 6 with a baking sheet or tray on the centre shelf. Divide the pastry into one-third and two-third pieces. On a lightly floured work surface, roll out the larger piece of pastry to about the thickness of a £1 coin and larger in diameter than needed to line a deep 20cm springform cake tin.

Step 7

Line the tin with the pastry, taking care to ease it gently into the corners. Leave the excess overhanging, then allow to rest for 10 minutes or so. Meanwhile, roll the remaining pastry to a circle of the same £1 coin thickness and about 24cm in diameter.

Step 8

Using a fork, prick the pastry base and spoon in the cooled filling. Brush the pastry around the edge with a little beaten egg, then carefully lay the circle of pastry over the top. Using a small knife, trim the excess pastry, leaving a slight excess, then crimp both together by pinching between your thumbs and forefinger.

Step 9

Brush the top of the pie generously with beaten egg and, using the small knife, cut 2 small holes in the centre. Use any excess pastry trimmings to make decorations or score a pattern, then brush with a little more beaten egg.

Step 10

Place the pie in the middle of the preheated tray, bake for 20 minutes, then reduce the temperature to 180ºC, gas mark 4 and cook for 45-50 minutes more, or until piping hot and golden. Allow to stand in the tin for 10 minutes, then release the sides. Serve with a generous portion of steamed greens and carrots with mash and gravy, if liked.