Preheat the oven to 200C.

Toss the chicken in the flour. Melt half the butter in a large frying pan and brown the chicken well. Remove and set aside. Melt the remaining butter in the same pan, add the onion, garlic and mushrooms and cook over medium heat for 5 minutes, or until the onion is soft. Add the stock and stir until the mixture boils. Return the chicken to the pan, add the sage and salt to taste (I used nearly 1 tbsp), stir, then turn the heat down and simmer for 5-10 minutes or until the mixture thickens. Remove from heat and allow to cool a little.

Grease a large, deep pie dish (roughly 20-25cm by 4cm deep - it doesn't need to be exact: simply trim the pastry to fit or join two pieces together if you need extra) and line with puff pastry, trimming around the edge. Spoon the chicken mixture into the pie dish and brush the edge of the pastry with egg. Lay another piece of pastry over the top and press the edges together firmly. Trim the pastry and make a couple of small cuts in the top for the steam to escape. Brush the top of the pie with more egg.

Place in the oven for about 30 minutes, or until the pastry is lightly browned and the filling cooked through.