1. Preheat the oven to 200C, 400F, Gas Mark 6. Divide the
pastry in two equal portions and roll each portion out to a
15cm/6-inch square on a well floured work surface.
2. Using the tip of a small sharp knife, score a border about
2.5cm/1-inch from the edges making sure you don't cut all the way
through the pastry. Place on a baking tray, brush well with beaten
egg and bake in the hot oven for about 15 minutes until well risen
and golden brown.
3. Meanwhile, heat the butter in a medium sized saucepan, add the
onions and fry gently until golden brown and softened, stirring from
time to time.
4. Add the stock and mascarpone cheese, stirring, then add the chicken,
salt and pepper and mix well. Partially cover the pan
and cook gently for 10
minutes stirring occasionally.