Heat 1 tablespoon of oil in a wide pan over high heat. Brown the chicken in two batches, then transfer to a plate.
Add the remaining oil and mushrooms. Cook 5-6 minutes until golden and their liquid has evaporated.
Reduce heat to medium. Melt the butter and cook the onion 6 minutes. Add the garlic and cook 1 minute.
Stir in the flour for 1 minute. Gradually add the stock, then the cream, tarragon and Dijon.
Return the chicken and mushrooms. Simmer gently 8-10 minutes. Add lemon juice, season, cool 15 minutes.
Preheat the oven to 200C fan. Spoon the filling into a 1.5-litre pie dish. Lay over the pastry, trim, crimp, cut a steam hole. Brush with egg.
Bake 25-30 minutes until deeply golden and bubbling. Rest 10 minutes before serving.
Tap to reveal the secret seasoningFresh tarragon – its gentle aniseed flavour lifts the creamy filling and makes a familiar chicken pie taste fresher and more refined.