Pre-heat oven to 375 degrees F.
In a large saucepan, heat some oil over medium heat. Saute onions, carrots and celery until translucent.
Add flour and butter to vegetables and cook for a few minutes.
Add stock and stir until thick.
Whisk in cream and bring to a gentle boil
Cook until desired consistency, using a whisk to avoid lumps.
Add chicken, peas and carrots.
Season with salt and pepper.
Pour into a greased casserole dish or individual dishes.
Cover with defrosted puff pastry. Brush top with beaten egg and cut a small "x" in the center.
Bake for 20 to 25 minutes, or until pastry is golden brown and bubbling.