Lighter creamy chicken pie

Name: 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