Sift the flour and a pinch of salt into a large bowl. Add the butter and use your fingertips to rub it into the flour until the mixture resembles crumbs.
Stir in the wholegrain mustard and 3-4 tbsp cold water and mix to form a firm dough. Alternatively whizz in a food processor to make a firm dough. Wrap and chill for 10 mins.
Preheat the oven to 220°C/Fan 200°C/Gas Mark 7.
Cut the pastry into six even pieces. Roll out each piece to a circle measuring about 14cm, then use to line a 12cm x 4cm fluted flan tin. Fill each tart with a piece of crumpled baking parchment then half fill with baking beans.
Stand the cases on a baking sheet and bake blind for 15mins removing the paper and beans for the final 5 mins until the base is golden.
Meanwhile prepare the filling: melt the butter in a medium pan, add the leeks and sauté for 4 mins. Stir in the nutmeg, season, cover and cook for 6-7 mins or until tender. Leave to cool. Beat the creme fraiche and eggs together with a little salt and plenty of ground black pepper.
Reduce the oven temperature to 200°C/Fan 180°C/Gas Mark 6. Divide the leeks between the flan cases, scatter over the stilton, then divide the creme fraiche mixture between them. Sprinkle over the parmesan and bake for 15-20mins or until the filling is set and golden. Cool in the tins for 5 mins before serving warm with a salad garnish.
If you prefer, use the pastry to line a 23cm fluted flan tin, bake blind and fill as before baking for 25-30 minutes or until the filling is set and golden. Serve warm in wedges. These tarts make great freezer standbys. Cool, then chill on the day of baking and freeze for up to 6 months.
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