Cook leek in butter in a large skillet over medium-high heat 3 minutes, stirring constantly. Add water, salt, and pepper. Cover, reduce heat, and simmer 10 minutes or until leek is tender. Uncover, increase heat to high, and continue cooking, stirring occasionally, 3 minutes or until liquid evaporates.
Bake piecrust at 350° for 9 minutes. Sprinkle cheese in bottom of piecrust; spoon leek mixture over cheese.
Whisk together eggs, egg whites, milk and cornstarch in a medium bowl; pour over leek mixture. Bake at 350° for 20 to 23 minutes or until set, shielding after 10 minutes, if necessary. Let stand 10 minutes.
Prepare leeks by first removing the root, the tough outer leaves, and the tops, leaving 2 inches of dark leaves. Wash the leeks thoroughly and slice thinly.