Preparation

Make the tart shell:

• In a food processor, pulse the flour, thyme, salt, and pepper to blend thoroughly. Add the butter and pulse until the butter pieces are about the size of rice grains (about eight 1-second pulses). Add the ice water 1 Tbs. at a time through the feed tube while pulsing in short bursts until the dough starts coming together. It may still look crumbly, but if you press it with your fingers, it should become compact. (Don’t add more water than absolutely necessary to get the dough to cling together.) Turn the dough out onto a clean work surface and, using your hands, gather and press the dough into a rough ball, blotting up the stray crumbs.

• Transfer the dough to a piece of waxed paper, shape it gently into a disk, and wrap it tightly to keep it from drying out. Refrigerate for at least 45 minutes. (The dough can be made up to 2 days ahead.) Position a rack in the center of the oven and heat the oven to 450°F. Unwrap the dough, set it on a lightly floured surface, and if necessary, let sit until pliable. Roll the dough out to a 14-inch circle about 1⁄8 inch thick.
• Transfer the dough to an 11-inch fluted tart pan with a removable bottom and press it carefully into the corners and up the sides of the pan. Let the edges of the dough hang over the rim of the pan and then roll the rolling pin over the top of the pan to cut away the excess dough. Prick the surface of the dough all over with a fork, line it with parchment, and fill it with pie weights or dried beans. Put the pan on a rimmed baking sheet and bake until the edges of the tart shell are dry and flaky (but not browned), about 10 minutes. Remove the weights and parchment; the center should still be moist and raw. Prick the bottom again and return the shell to the oven. Bake until the bottom surface is completely dry, 5 to 7 minutes more. Remove from the oven and let cool.

  Lower the oven temperature to 375°F.

Make the filling:

• In a 12-inch skillet, cook the bacon over medium heat until it’s crisp and golden brown, about 5 minutes. With a slotted spoon, transfer the bacon to a dish and set aside. Discard all but about 2 tsp. of the bacon fat. Set the skillet over medium-low heat, add the butter, let it melt, and then add the leeks. Stir to coat them with the fat, cover, and cook, stirring occasionally, until soft, 8 to 10 minutes. Stir the flour into the leeks and cook uncovered, stirring, for about 2 minutes to cook off the raw-flour flavor. Set aside and let cool slightly. In a medium bowl, lightly whisk the eggs. Add the cream, milk, salt, nutmeg, and several grinds of pepper and whisk until blended. Add the bacon and leeks to the mixture and stir to combine.
• To assemble the tart, scatter 1⁄3 cup cheese over the cooled tart shell and pour in the egg mixture. Spread the leeks evenly. Scatter the remaining 1⁄3 cup cheese evenly over the top. Bake until the custard is set and the top is light golden brown, about 35 minutes. Let cool on a rack for at least 30 minutes before serving.

Store leftovers in the refrigerator, covered. Reheat for 10 to 15 minutes at 350°F.