Preheat the oven to 475 F with the rack set in the middle position. Place all of the tart pastry ingredients into the bowl of a food processor fitted with a metal blade and process until the mixture blends together to form a mass, about 20 seconds. If it doesn’t form a mass after 25 seconds, add another teaspoon of water.
Turn the pastry out onto a lightly floured work surface, dust the pastry lightly with flour and begin the blending or kneading process known as “fraisage”: With the heel of your hand, push the pastry down and away from you a little at a time and repeat this process three or four times or until the pastry is smooth and does not stick to the work surface.
Lightly dust the pastry with flour and shape into a flat round, much like a thick hamburger. The pastry can be used immediately, unless it is too warm and soft. In which case, wrap it in plastic wrap and refrigerate for 10 to 20 minutes. Line a 91⁄2- or 10-inch tart pan with removable bottom with the pastry.
Prick the pastry, line it with aluminum foil, and weight it with 1 pound of dried rice or aluminum pie weights. Bake until the edges of the pastry begin to color, about 20 minutes. Remove the pan from the oven and remove the foil and beans from the pan. Spread the mustard over the bottom of the pastry. Lower the oven temperature to 425 F. In a bowl, beat the eggs and yolk lightly. Blend in the milk, salt, pepper, and nutmeg.
Spread the ham and cheese evenly over the bottom of the tart shell. Pour the custard into the tart shell to within 1⁄8-inch of the top of the crust. Holding the tart pan by the outer rim (so as not to dislodge the bottom), place the pan in the oven. Bake until the custard puffs, 25 to 30 minutes. Unmold as soon as possible. Allow to cool for at least 10 minutes before cutting into wedges and serving. The quiche is delicious served at any temperature.