This easy cheese-and-vegetable recipe makes enough for one quiche baked in a nine-inch springform pan, or two quiches baked in eight- or nine-inch pie plates.
Crust: Pulse flour and salt in a food processor until combined. Add butter and pulse until mixture resembles coarse meal. Whisk together egg and yolk with ice water. Add to flour mixture and pulse until moist crumbs start to form (no longer). Transfer mixture to a piece of plastic wrap and, using wrap and your hands, form into a disk (or 2 disks, if making 2 quiches in pie plates). Refrigerate at least 30 minutes and up to 4 hours.
Let sit at room temperature until pliable, then roll out dough on a lightly floured surface to a 14-inch round. Fit into a 9-inch springform pan, folding and pressing sides to an even thickness. (Or roll out 2 disks to 11-inch rounds and fit into 8- or 9-inch pie plates.)
Trim edge of dough flush with top of springform pan. (If using pie plates, fold edges of dough under; crimp as desired.) Wrap in plastic; freeze at least 30 minutes and up to 3 days. Preheat oven to 400 degrees.
Line crust with parchment; fill with dried beans or pie weights. Bake, with a baking sheet on rack below, 20 minutes; remove paper and beans. Bake until golden, 12 to 15 minutes (8 to 10 minutes, for pie plates). Let cool.
Reduce oven to 375 degrees. Whisk eggs until smooth, then whisk in remaining ingredients until well combined.
Pour custard mixture into cooled crust, stopping just short of top (if making in 2 pie plates, divide evenly between crusts). You may not need to use all of custard; be careful not to overfill. Press spinach into custard(s), leaving some poking out from top.
Sprinkle with Gruyere. Bake 10 minutes at 375 degrees, then about 1 hour at 325 degrees. Let cool about 1 hour before serving.