Preheat the oven to 425.
In a small saucepan, make the filling: add the butter to the pan and let it melt. Add the chopped vegetables and saute (while stirring occasionally) for about 5 minutes.
Sprinkle the flour over the vegetables, and cook for 30 seconds while stirring.
Add the stock, milk, thyme, salt and pepper. Let this mixture come to a simmer. Taste and adjust for seasoning.
Finally, stir the cooked chicken into the mixture and remove from the heat.
Ladle the chicken filling mixture into the serving dishes.
Remove the pastry from the packaging, and unroll it. Cut it in half, and place the remaining half back in the box for another use. Roughly roll out to cover both serving bowls.
Cut out the pastry circles, and place one on top of each bowl. It's best if the filling doesn't touch the pastry, but it's totally fine if it does. Beat the egg yolk, and brush it over the crust.
Bake for 15-19 minutes, until the filling is bubbling and the crust is golden brown.
Remove from the oven and let cool for a few minutes before serving.
