Make crust: In a food processor, pulse flour, sugar, and salt until combined. Add butter and pulse until mixture resembles coarse meal, with a few pea-size pieces of butter remaining. Sprinkle with 1/4 cup ice water. Pulse just until dough is crumbly but holds together when squeezed (if necessary, add 1/4 cup more ice water, 1 tablespoon at a time). Do not overmix. Form dough into a disk, wrap tightly in plastic, and refrigerate until firm, 1 hour (or up to overnight).
Preheat oven to 375 degrees. On a lightly floured work surface, roll out dough to a 12-by-16-inch rectangle (patch any holes well). Place in a 10-by-14-by-1-inch rimmed jelly-roll pan with dough going up the sides; trim overhang. Prick dough all over with a fork and freeze until firm, 20 minutes. Line dough with parchment paper, leaving overhang on all sides. Fill with pie weights or dried beans. Bake until edges are dry, about 15 minutes. Remove parchment and weights; bake until bottom is dry and flaky, about 10 minutes.
Make filling: In a medium bowl, whisk together citrus zests and juices, granulated sugar, and eggs until well combined. Pour into crust and bake until custard is just set in center, about 15 minutes. Let cool completely in pan on a wire rack, 1 hour. To serve, dust with confectioners' sugar and cut into 18 squares.
Refrigerate, tightly wrapped in plastic, up to 3 days.