In a large bowl, combine the flour with butter, egg, salt and 1 tablespoon of the water. Mix lightly with your fingertips until pastry forms peasized pieces. You should be able to see chunks of fat, and the pastry should be moist enough to begin to stick together. If the pastry is too dry, add up to 2 more tablespoons of water.
Turn the pastry out onto a lightly floured work surface, dust with flour, and knead it until the pastry is smooth, about 3 to 4 times. Transfer to a plastic bag and form pastry into a disk. Refrigerate a minimum of 30 minutes, or as long as 3 days.
Cook bacon in a skillet over low heat until tender, not crisp. Add leeks, salt and pepper, and saute until leeks are soft and bacon is crisp, 7 to 10 minutes. Drain off excess fat, if any. Set aside.
Preheat the oven to 375 degrees F and arrange the rack in the middle of the oven. On a generously floured surface, roll pastry from center out, lifting pastry, turning it slightly, and occasionally flipping to prevent sticking. Roll pastry to 1/8inch thickness. Lightly butter and flour a 9inch pie pan and line with pastry, leaving about 1/4inch overhang for shrinkage. Pinch up excess pastry to form a rim and flute the edges. Prick the bottom all over with a fork.
Line pastry with a sheet of parchment or aluminum foil larger than the pan, and fill with pie weights, rice, or beans. Bake about 20 minutes, until the edges begin to color. Remove the paper and weights, and spread the mustard over the bottom of the pastry. Lower the oven temperature to 325 degrees F.
In a bowl, beat the eggs lightly with the milk. Stir in bacon and cheese mixture. Pour the custard into the pie shell to within 1/4inch of the top of the crust. Bake 25 to 30 minutes, or until custard puffs. Allow to cool 10 minutes before serving. Serve warm or room temperature.
c.1997, M.S. Milliken & S. Feniger, all rights reserved