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Fry the pancetta in a hot, dry pan for 3–4 minutes until lightly golden. Tip onto a plate lined with kitchen paper. Wipe out the pan.
Heat the oil in the pan and sauté the vegetables with the thyme and some seasoning for about 7 minutes until softened. Pour in the sherry and bubble for 5 minutes until well reduced. Return the pancetta to the pan and pour in the stock.
Return the liquid to the boil and cook for 10–15 minutes until reduced by half. Stir in the crème fraîche and cook for about 5 minutes until the sauce has reduced by about a third. Check for seasoning, then remove from the heat and set aside.
Preheat the oven to 200°C/Gas 6. Roll out the pastry on a lightly floured surface to the thickness of a £1 coin. Cut out 2 or 3 rounds, using a small saucer as a template. Carefully place the pastry rounds on a large, non-stick baking sheet and score the surface in a diamond pattern, using the tip of a small sharp knife.
Brush the pastry with the egg yolk glaze and bake for about 10 minutes until risen and golden. Bake for a further 2 minutes with the oven door slightly ajar, to help crisp the pastry. Remove from the oven and slide onto a wire rack.
Meanwhile, add the chicken to the sauce and reheat until the meat is warmed through. Check the seasoning and divide the chicken mixture between your warmed serving plates. Top each pile with a pastry round and serve.