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Preheat the oven to 200C /Fan 180C /Gas Mark 6. First make the pastry: sift the flour, mustard, cayenne and salt into a large bowl. Rub in the butter until the mixture resembles fine crumbs, then stir in the cheese. Add 45-60ml (3-4tbsp) cold water and mix to a firm dough. Lightly knead, then wrap in cling film and chill for 10 minutes.
Meanwhile, prepare the filling. Melt the butter in a frying pan, add the leeks and saute for 2 minutes or until tender. Blanch the asparagus spears in a large pan of boiling water for 2-3 minutes or until tender. Drain, rinse in cold water and drain again. Beat the eggs and creme fraiche together and season well.
Roll out the pastry and use to line a 20cm/8in fluted flan tin. Line the case with baking parchment and half fill with baking beans. Bake for 20 minutes, removing the paper and beans for the final 5 minutes.
Reduce the oven to 180C /Fan 160C /Gas Mark 4. Scatter half the cheese over the pastry base, top with the leeks and asparagus. Pour over the egg mixture then scatter over the remaining cheese. Bake for 30-35 minutes or until the top is golden and filling set. Serve warm with salad.