Prepare the Pastry: In a food processor, combine plain flour and salted butter, pulsing until the mixture resembles fine breadcrumbs. Add a large egg and cold water gradually, mixing until the dough forms clumps. Turn the dough onto a lightly floured surface, shape it into a disc, wrap in cling film, and chill in the fridge for at least 1 hour.

Cook the Filling: In a large skillet over medium heat, cook diced bacon until crispy, about 5-7 minutes, then transfer to a bowl. In the same skillet, add diced boneless skinless chicken thighs, browning them for about 5 minutes. Remove the chicken and deglaze with apple cider, then sauté chopped brown onion, carrots, celery, and fresh thyme for 5-6 minutes until tender.

Thicken the Filling: Return the chicken and bacon to the skillet, adding a sprinkle of plain flour, chicken stock, thickened cream, and Dijon mustard. Stir well, bringing the mixture to a simmer for about 3-4 minutes until it thickens. Remove the skillet from heat and allow the filling to cool slightly.

Assemble the Pie: Preheat your oven to 200°C (400°F). Roll out the chilled pastry to fit your pie dish. Lay the pastry in the dish, filling it with the cooled chicken and bacon mixture. Roll out a second layer of pastry for the top, crimp the edges to seal, and brush with a beaten egg for a golden finish. Poke vent holes for steam to escape.

Bake the Pie: Transfer the Chicken and Bacon Pie to the preheated oven and bake for 50-60 minutes, or until the pastry is golden brown and flaky. Let the pie rest for 10 minutes before slicing.