Lighter creamy chicken pie

Ingredients

• 1 tbsp sunflower oil
• 3 boneless, skinless chicken breasts (around 500g/1lb 2oz), cut into roughly 2cm/¾in chunks
• 1 onion, finely chopped
• 300g/10½oz chestnut mushrooms, halved or quartered if large
• 4 tsp plain flour
• 1 tsp garlic powder or 1 garlic clove, crushed
• 1 tsp dried tarragon or parsley
• 400ml/14fl oz chicken stock, made with 1 stock cube
• 4 tbsp reduced fat crème fraîche
• 2 x sheets filo pastry (around 85g/3oz)
• freshly ground black pepper