Preheat oven to 450°. Fit piecrust into a 9-inch pie plate according to package directions; fold edges under, and crimp.
Bake at 450° for 8 to 10 minutes or until golden brown. Remove from oven, and let cool 5 minutes. Reduce oven temperature to 375°.
Cut tomatoes into 1/4-inch slices. Place tomatoes on a paper towel-lined wire rack. Sprinkle tomatoes with kosher salt. Let stand 20 minutes. Pat dry with paper towels.
Remove and discard root end and dark green top of leek. Cut in half lengthwise, and rinse thoroughly under cold running water to remove grit and sand. Thinly slice leek.
Melt butter in a large skillet over medium heat; add leek, and sauté 3 to 5 minutes or until tender.
Layer leek on bottom of prepared crust. Top with tomato slices, and sprinkle with pepper. Stir together cheese, mayonnaise, and egg in a medium bowl until blended. Spread cheese mixture over top of tomatoes.
Bake at 375° for 30 minutes or until thoroughly heated. Let stand 10 minutes. Garnish, if desired.