Pre-heat oven to 190°C / 170°C fan assisted / gas mark 5 / 375°F.
Peel (if necessary) and dice the onion, celery, fennel and leek into small 1 cm pieces.
Melt 1 tablespoon of the butter with the olive oil.
Add the onion, fennel, leek and celery and cook for 10-15 minutes on a medium-low heat until soft and just starting to catch.
Add the turkey meat, garlic, herbs and ½ teaspoon of the salt and pepper and gently stir to mix.
Sprinkle the flour over then mix to make sure all the fat has been absorbed.
Slowly pour in the stock, using a spoon to stir in, so the sauce is created and thickens slowly.
When all the stock has been added and the sauce is thick, smooth and just starting to bubble then stir in the crème fraiche until it melts evenly into the sauce.
Remove from heat and pour the turkey filling into a large ovenproof dish.
Peel and slice the potatoes thinly, use a mandolin if you have one. Then layer over the turkey, dotting in the extra butter and salt between each layer and over the final layer. There should be about 3 layers of potatoes.
Bake for 85-90 minutes until crisp and golden.