For the sauce, melt the butter in a heavy-bottomed saucepan, add the flour and cook slowly over a low heat for 2-3 minutes. Gradually pour in the chicken stock, turn up the heat to bring to the boil, stirring constantly and reduce to a gentle simmer. Cook for 4-5 minutes, add the cream and sage and continue to simmer for 2-3 minutes. Add the onion, turn off the heat and set aside to cool for 10-15 minutes.
Cut the vegetables into bite-size pieces and add to a large bowl with the chicken. Pour over the sauce (discard the onion) and mix well.
Brush the edge of a 20-22cm pie dish with beaten egg, unroll the pastry and with a sharp knife cut strips of pastry (approx 2cm wide) from the edges and use to form a rim on the pie dish. Brush the edge with beaten egg to help the pastry stick.
Carefully lift the remaining pastry, lay over the dish and press down the edges to form a good seal.
Trim off any excess pastry with a sharp knife (reserve the trimmings to decorate) and flute the edges. Make two holes in the centre of the pie to allow the steam to escape.
Re-roll the trimmings and cut out leaves or other shapes of your choice. Brush the pastry top with egg wash, arrange leaves on top and brush again. Bake for approximately 25-30 minutes until the pastry is golden brown.