Preheat oven to 400 degrees. Roll out pate brisee into a 1/4-inch-thick rectangle on a lightly floured surface. Fit dough into a 4 1/2-by-13 3/4-inch rectangular tart pan (available at williams-sonoma.com), leaving a 2-inch overhang on all sides. Fold under overhanging edges, pressing firmly against pan, to double thickness of crust sides. Prick bottom with a fork. Chill in freezer for 30 minutes.
Line dough with parchment cut to fit, and top with pie weights or dried beans. Bake until edges are golden, about 20 minutes. Remove pie weights and parchment, and bake until bottom is golden, 10 to 15 minutes more. Let cool in pan on a wire rack. Reduce oven temperature to 350 degrees.
Meanwhile, saute leeks in butter over medium-high heat, stirring frequently, until leeks are just tender, 4 to 5 minutes. Transfer leeks to a bowl, and let cool slightly.
Add eggs and yolk, cream, creme fraiche, cheese, herbs, and salt to leeks, and stir to combine. Season with pepper. Spoon filling into cooled tart shell. Bake until filling is set in the center, 30 to 35 minutes. Let cool in pan on wire rack. Remove from pan and cut into squares.
Comte cheese is available at specialty-food stores and some grocery stores. Gruyere cheese, which is similar in texture but a bit stronger in flavor, may be substituted.