Melt the butter in a deep frying pan and add the onion. Cook over a gentle heat, stirring constantly until the onion is just soft.
Add the mushrooms and continue stirring until the mushrooms are just cooked. Add the garlic and flour and carry on cooking for another minute before adding the milk and stock, a little at a time, stirring to avoid lumps.
Stir in the crème fraiche, bay leaves and thyme and season well. Add the turkey and vegetables. Transfer to an oven proof dish.
Lay over the sliced potatoes, sprinkle over the cheese and drizzle with olive oil.
Bake in the oven for 30 minutes until hot and bubbling.