For the pastry, sift the flour and salt into the bowl of a food processor, add the butter and lard, then whizz together briefly until the mixture looks like fine breadcrumbs.
Tip the mixture into a bowl, then stir in the cheese and enough of the water for the mixture to come together.
Tip out onto a lightly floured surface and knead briefly until smooth.
Roll out thinly and line a 23cm x 4cm loose-?bottomed fluted flan tin.
Prick the base with a fork.
Chill for 20 minutes.
Melt the 75g butter in a saucepan over a low heat, then add the leeks and the salt.
Cover and cook for ?10 minutes until soft.
Uncover the pan, increase the heat and cook ?for 2 minutes, stirring occasionally, until the liquid has evaporated.
Spoon onto a plate and leave to cool.
Preheat the oven to 200°C/fan180°C/gas 6.
Line the pastry case with baking paper and baking beans or rice and blind bake for 15-20 minutes until the edges are biscuit-coloured.
Remove the paper and beans/rice and return the case to the oven for 7-10 minutes until the base is crisp and lightly golden.
Remove and set aside.
Reduce the oven temperature to 190°C/fan170°C/gas 5.
Put the crème fraîche into a bowl with the whole egg, egg yolks and nutmeg.
Lightly beat together, then season.
Stir in the leeks.
Spoon ?the mixture into the tart case and bake for 35-40 minutes until set ?and lightly golden.
Remove from ?the oven and leave for 10 minutes.
Take out of the tin and serve.