Instructions

• Make the Crust: Combine 4 T ice water and the cider vinegar in a small bowl and set aside. Blend flour and ¾ tsp salt in a food processor. Add 9 T butter (cut into small cubes) and cut in using on/off turns until the mixture resembles coarse meal. With the machine running, slowly add water-vinegar mixture, processing until moist clumps form. If dough seems dry, add ice water by the teaspoonful (I usually end up adding ~2 more tsp).
• Gather dough into a ball, flatten into a disk, wrap in plastic and refrigerate at least 2 hours (you can make this up to 3 days ahead; simply keep refrigerated and let soften slightly at room temp before rolling out).
• Position rack in center of oven and preheat to 375. Roll dough out (on waxed paper or a lightly floured surface) to a 12″ round. Transfer to 9″-diameter tart pan with removable bottom. Press dough onto bottom and up sides, folding in overhang and extending dough 1/2″ above sides of pan. Line pan with foil and dried beans or pie weights. Bake until dough looks set and dry, 25-30 minutes. Remove foil and beans and continue to bake until crust is pale golden, 15-20 minutes longer. Remove from oven and cool while preparing rest of tart.
• Make the Leek Confit: Melt 4 T butter in a large pot over medium-low heat. Add leeks and stir to coat. Stir in 2 T water and ½ salt. Cover pot and reduce heat to low. Cook until leeks are tender, stirring often, about 25 minutes. Uncover and cook to evaporate excess water, 2-3 mins. (You can make the confit up to 1 week ahead; keep chilled and rewarm before using). This makes about 2 c of confit, and you’ll need just 1½ c for this tart. Use the rest in eggs, on salmon or on crostini.
• Finish the tart: Whisk milk, cream, egg, egg yolk, and salt in a medium bowl until blended. Sprinkle ¼ c cheese over bottom of warm crust; spread leek confit over, and sprinkle with remaining cheese. Pour milk mixture over. Bake until filling has puffed, is golden in spots and center looks set, 35-40 mins.
• Transfer to a rack and cool slightly. Remove pan sides; serve warm or at room temp.