Combine flour, butter, and salt on a work surface by hand until grainy.
Mix in ice water until incorporated, maintaining a marbled effect.
Roll into a 2.5cm thick rectangle and refrigerate for 20 minutes.
Roll out to a 40x20cm rectangle on a floured surface.
Fold the pastry into thirds and rotate 90 degrees.
Repeat rolling and folding twice more, resting in fridge between turns.
Refrigerate the finished pastry for 30 minutes before use.
Preheat your oven to 200°C.
Fry shallot, garlic, and chicken in butter in an ovenproof pan.
Add mushrooms and fry, then add Madeira and flambé carefully.
Simmer with stock, tarragon, and cream for five minutes then cool.
Cover the filling with pastry, brush with egg, and crimp edges.
Bake for 25 minutes until the pastry is golden and crisp.