In a food processor, pulse together plain flour and chilled salted butter until coarse crumbs form. Add a beaten egg and cold water, blending until a dough ball forms. Wrap in plastic wrap and refrigerate for at least 1 hour.

Heat a large skillet over medium heat, add vegetable oil. Once hot, add diced bacon and chicken thighs, cooking until slightly browned (about 5-7 minutes). Remove from pan, deglaze with apple cider.

In the same pan, add diced onion, carrot, and celery, stirring until softened (about 4-5 minutes). Add fresh thyme and flour, stirring to coat the vegetables.

Pour in chicken stock and thickened cream, stirring to combine. Bring to a low simmer and thicken (about 3-4 minutes). Return cooked bacon and chicken, mix well, and let cool slightly.

Roll out the larger piece of pastry and line a greased pie dish. Spoon cooled chicken and bacon mixture into the crust, packing gently. Roll out the second pastry sheet to cover, crimping edges.

Preheat oven to 200°C (400°F). Brush top with egg wash and create holes for steam. Bake for 50-60 minutes until crust is golden brown and flaky.