Heat 1 tablespoon of the oil in a large non-stick frying pan over a medium-high heat. Season chicken then cook, in two batches, for 5 minutes or until brown. Remove from pan. Add 2 tablespoons of oil to same pan. Add mushrooms and cook, stirring, for 5 minutes or until light brown. Remove from pan.
Meanwhile, heat remaining oil in a large cast iron or heavy based pot over a medium heat. Add celery, onions and garlic and cook, stirring, for 5 minutes until soft.
Add butter and stir until melted. Add flour and cook, stirring, for 2 minutes. Remove from heat. Add ½ cup of stock and stir until sauce is free of lumps. Gradually add remaining stock and stir until combined.
Return to heat and bring to the boil. Stir in chicken, mushrooms, wine and bay leaves and simmer for 20 minutes, stirring occasionally, until thickened. Remove from heat and cool for 15 minutes. Stir in sour cream.
Meanwhile, make pastry; put flour, butter and gruyere in a food processor and process until mixture resembles breadcrumbs. Add sour cream and process until mixture comes together. Turn out onto a floured surface and bring together to form a ball. Knead lightly until smooth. Cover with plastic wrap, pat into a disc and refrigerate for 30 minutes.
Preheat oven to 180°C. Spoon chicken mixture into a 26cm ceramic/glass or metal pie dish. Scoop out bay leaves and discard.
Roll out pastry on a floured surface until 7mm thick and large enough to cover pie dish. Wet edge of dish. Cover pie with pastry, then trim edges. Cut a cross in centre of pastry for venting. Bake 20-25 minutes or until golden. Serve.