Preheat the oven to the puff pastry instructions on your packet. Heat the oil in a saucepan or frying pan and add the chicken and mushrooms. Season and pan fry for about 10 minutes on medium heat.
Meanwhile, heat the butter in a small saucepan over low heat. When melted add the flour and stir quickly and constantly with a wooden spoon. After a few minutes, slowly add the milk a little at a time. You should end up with a thickish sauce. Remove from the heat and stir in the cheese and seasoning.
In a bowl, combine the chicken mixture and the sauce. Cover and set aside to cool down. This can be made ahead 24 hours if required and brought to room temperature before using.
On a floured surface, cut the puff pastry block into four. Roll out each into a square approx 0.5cm thick. On two of the squares evenly distribute the filling ensuring you keep a gap around the edges of about 3cm. Brush around the edge with beaten egg. On the remaining pieces, brush the edges and place on top of the filling. Press down all around the edges and with a knife trim away to obtain a circular shape. Crimp around the edges. Brush all over with beaten egg. (With the remaining pastry you may want to make a decorative shape, I have made a heart. Brush your decoration also with egg.)
Grease a baking tray and place your puff slices onto it. Bake for 25 to 30 minutes at 220 C / Gas 7, or use your pastry packet for guidance.
Make the chicken and mushroom mix and the cheese sauce 24 hours in advance for an even easier and quicker option.
This was a big success for Valentine's Day dinner. I added onions and garlic when cooking the chicken and before adding the mushrooms. Added the pastry heart on top, just like the pic. I was very happy that the pastry was not soggy on the bottom after baking. - 14 Feb 2012