Sift flour and salt into a bowl, rub in butter until it looks like breadcrumbs, then add water to form a dough.
Knead the dough until smooth, wrap in film, and chill in the fridge for 10 minutes.
Roll dough into a 20cm x 35cm rectangle, fold into thirds, and chill for another 10 minutes.
Turn the dough 90 degrees and repeat the rolling/folding/chilling process four more times.
Melt butter in a pan and brown the chopped chicken breasts on all sides.
Stud the onion halves with cloves and add them to the pan with mushrooms and garlic.
Fry for a few minutes, season with salt and pepper, and pour in the white wine.
Boil until the wine reduces, then stir in the cream and simmer for 10 minutes until chicken is cooked.
Transfer the chicken mixture to a 23cm pie dish and brush the edges with water.
Preheat the oven to 200°C.
Roll out the pastry, drape over the dish, trim the edges, and press firmly to seal.
Bake for 25 to 30 minutes until the top is golden-brown.