Place the flour and butter into a food processor and pulse until the mixture looks like breadcrumbs. Add the egg and water and pulse until it just begins to ball up.
Roll out on a lightly floured surface and transfer to a 11-in/28cm deep quiche pan. Press up along the rim them prick with a fork. Chill for 15 minutes.
Line the pastry with parchments and pie weights (I use dry beans) and bake for 15 minutes. Remove the weights and paper and bake for an additional 5-10 minutes or until just cooked and pale golden. Remove crust from the oven. Reduce oven temp to 350F/180C.
To make filling: heat the oil in a large skillet then add the leeks and celery and saute over high heat for a few minutes.
Lover the heat, cover and simmner for 25 minutes or until veggies are tender. Remove cover and increase heat to high to evaporate any accumulated liquid.
Beat the eggs and cream together in a medium bowl. Whisk in the salt and pepper.
Spread the cooked leeks and celery evenly on the bottom of the crust. Top with the cheese and parsley. Pour the custard over the top.
Bake for 35-40 minutes or until golden and just set on top. Let the quiche stand for 5-10 minutes before cutting and serving.
Gorgonzola or bleu cheese may be substituted if you cannot find Stilton.