Fry the lardons (or chopped ham) in a dry frying pan for 3 minutes until lightly golden, drain onto a plate and set aside.
In a sauté pan, add the olive oil and gently sauté the leeks, parsnips and butternut squash with the thyme leaves and salt & pepper until soft (8 minutes), being careful not to burn them (as this will affect the flavour).
Pour in the sherry and let it bubble for 5 minutes until well reduced.
Add the lardons (or chopped ham) and pour over the chicken stock.
Return to the boil and simmer for about 10 minutes until the liquid is reduced by half.
Stir in the crème fraiche and cook for 5 minutes until the sauce has reduced down to nice, thick, creamy consistency.
Pre-heat the oven to 200C.
Roll out the puff pastry to 3mm thick on a floured surface.
Cut out 2 rounds using a small saucer as a template (about 12cm diameter) and then 2 slightly smaller ones.
Place the pastry circles on a non-stick baking tray, with a small one on top of a large one and score a pattern (of your own design) on the top of each with a knife.
Brush the top of the pastry circles with the egg and water mixture and bake in the centre of the oven for 10 mins or until well-risen and golden brown.
Remove from the oven and slightly cool the pastry circles on a wire rack.
Add the cooked chicken to the sauce and re-heat until the chicken is properly heated through.
Check the seasoning and then divide the mixture between 2 warm plates, topping each one with a pastry circle.
Serve the pie with green beans, broccoli or peas and a large glass of white wine.