Melt the butter in a large, non-stick, heatproof dish or pan over medium heat on the hob.

Add the onions and fry for 5 minutes until soft, stirring often.

Add the chicken and fry for 5-10 minutes until browned, stirring often.

Stir in the mushrooms and cook for a few minutes, stirring often until they’ve reduced slightly.

Add the potatoes.

Stir in the flour, mustard, tarragon and pepper.

Add the stock, stirring in a bit at a time.

Pour in the cream and stir well.

Bring to a simmer.

Reduce the temperature if necessary and simmer for 30 minutes, stirring occasionally.

After 30 minutes, remove from the heat and leave to one side to thicken a little.

On a lightly rolled surface, roll the pastry into a square big enough to cover your pie tin and the filling comfortably.

Spoon the pie filling into your tin, filling as much as possible.

Place the pastry square on top and pinch the edges into the lip of the tin in a crimping style to seal it firmly and hold the pastry in place.

Liberally brush with whisked egg.

Pierce the top with two holes.

Carefully place it in the air fryer basket.

Air fry at 180 degrees C for 20 minutes until the pastry is golden brown and the chicken is piping hot. I recommend using a meat thermometer to make sure the chicken is above 75 degrees C.

If it needs longer, reduce it to 160 degrees C and check every five minutes.

Carefully remove from the air fryer and serve.