Combine the flour, salt and butter the bowl of an electric mixer with a paddle attachment and mix until the butter is crumbled into small pieces, about the size of peas. Slowly add only enough water to make the dough come together. Remove the dough and gently roll into a ball and wrap in plastic wrap to let rest 20 minutes in the refrigerator.
Remove the dough from the refrigerator and allow to come to room temperature for another 15 or so minutes.
Roll out the pate brisee dough to about 1/4-inch thickness. Use some bench flour to keep things from sticking.
Cut circles in the rolled-out dough. Fit the dough into individual tart rings on a baking sheet. Or using a paring knife, cut small disks in to the rolled-out dough and place into greased cupcake tins.
Into each uncooked tart shell divide evenly a little piece of the cheese and some of the sauteed leeks.
Add the custard enough to come to 1/4-inch from the top of the tart shell. Brush the tart shell with the egg wash.
Bake for 15 to 20 minutes or until golden brown and custard has set. Serve warm or room temperature.