Place the potatoes in a saucepan and cover with cold water. Bring to the boil. Simmer for 10-12 minutes or until just tender. Drain then set aside to cool slightly. Peel and quarter the cooled potatoes.

Meanwhile, heat half the oil in a large frying pan over medium-high heat. Add half the chicken and cook, turning occasionally, for 2-3 minutes or until browned. Transfer to a plate. Reheat pan then repeat with remaining oil and chicken.

Add the bacon, mushroom and leek to the frying pan. Cook, stirring occasionally, for 5 minutes or until the leek softens and the bacon is slightly browned. Add the garlic and cook, stirring, for 30 seconds or until combined. Pour in the wine and cook for 2-3 minutes or until reduced by half. Add the flour and cook, stirring, for 30 seconds. Remove from the heat.

Gradually stir stock into the bacon mixture until well combined. Return the chicken to the pan and add the potatoes and thyme. Cook over medium heat, stirring occasionally, for 3-4 minutes or until the mixture thickens and comes to the boil. Stir in the crème frâiche and mustard then remove from heat. Set aside for 1 hour to cool slightly. Transfer to a bowl and cover with plastic wrap. Place in the fridge for 2 hours to cool completely.

Preheat oven to 220°c/200°c fan forced. Spoon the mixture into a 2L (8 cup) square or rectangular ovenproof dish.

Cut each pastry sheet into 9 squares (you will need 16, so reserve any leftover pastry squares for another use) and brush with the egg yolk. Arrange pastry squares on top of the chicken mixture, overlapping each square and the edges of the dish. Bake for 20-25 minutes or until puffed and golden. Sprinkle with extra thyme, season and serve.