Keep the pan over medium heat and add the pancetta into the dry pan. Cook it for about 3-4 minutes until lightly brown. Then, take the pancetta out of the pan and place it on a plate lined with kitchen paper. Wipe out the pan clean.
Add oil to the same pan and heat it. Add the leek, celeriac, and butternut squash to the pan. Saute them for some time and sprinkle thyme, salt, and pepper over them. Cook the vegetables for 6-7 minutes until they are softened.
After that, add the sherry to the pan and bring it to a boil. Boil it for 5 minutes until it is well reduced. Return the pancetta to the pan and pour the chicken stock into the pan as well.
Bring this mixture to a boil and continue to do so for 10-15 minutes. When it is reduced to half, add the crème fraîche to the pan and cook for about 5 minutes.
When the sauce has reduced to one-third of its volume, remove the pan from heat.
Then, preheat the oven to 200°C.
Now, roll the puff pastry on a floured surface to a thickness of 2.5 mm. Cut about 4 rounds from the rolled-out pastry of about 6-inch diameter.
Place the rounds on the baking sheet, score them with a knife, and brush them with egg wash. Then, bake these pastry rounds for 10 minutes in the oven. Open the oven door and bake them for 2 more minutes so as to crisp them. When baked, place them on a wire rack to cool down.
For serving, warm up the sauce and add the cooked chicken to it as well. When the sauce and chicken are warm enough, place them on a plate and top with the baked pastry.
Your delicious chicken and vegetable pies are ready.