Ingredients:
Crust
2/3 cup(84 grams) all-purpose flour, plus more for the surface
2/3 cup(79 grams) white whole-wheat flour (see Author Notes)
1/2 teaspoonkosher salt
1/2 teaspoonfreshly ground black pepper
1/8 teaspoonbaking powder
3 ouncescream cheese, very cold, cubed
4 ounces(1 stick) unsalted butter, very cold, cubed
1 1/2 tablespoonswater, very cold
1 1/2 teaspoonsapple cider vinegar
1 large egg white
Filling & Assembly
1 tablespoonextra-virgin olive oil
1 (6-ounce) yellow onion, finely chopped
1 1/4 teaspoonskosher salt, plus more
2 garlic cloves, finely chopped
6 ouncesbaby spinach
1 large egg white
1 2/3 cupshalf-and-half (see Author Notes)
3 tablespoonsall-purpose flour
4 large eggs
1 large egg yolk
1/2 teaspoonfreshly ground black pepper
1 cupcoarsely grated Gruyère, divided
Directions:
Make the crust: In the large bowl of a stand mixer fitted with the paddle attachment, combine the all-purpose flour, white whole-wheat flour, salt, black pepper, and baking powder. Mix on low speed just to combine. Add the cream cheese and mix on low speed until the cream cheese is mostly broken down. Add the butter and mix on low speed until mostly incorporated and none of the pieces are bigger than a chickpea; this should take 30 seconds to 1 minute (if there are any stragglers, just stop the machine and smush the piece with your fingertips—don’t overmix). In a small glass, combine the water and vinegar. With the motor running on low speed, drizzle in the vinegar mixture. As soon as the sides of the bowl are no longer dusty and clumps and curds begin to form (it should not form a cohesive ball—that’s too far), turn the mixer off; better to undermix versus overmix at this point.
Remove the bowl and bring the dough into a ball with your hands. Wrap well with plastic or a reusable wrap and form into a flat disc. Refrigerate for at least 1 hour or up to a couple days. (You can also freeze the dough for future quiches; just thaw in the fridge overnight.)
Roll out and crimp the crust: On a floured work surface, roll out the dough into a 12-inch or so circle, then transfer to a 9-inch pie pan (not deep-dish), making sure the bottom corners are tightly pressed against the pan. If needed, use scissors to trim the dough to an even 1-inch overhang. Fold the edge under itself. Using your fingers, press and even out the sides, making sure the bottom corners are tightly pressed against the pan. Crimp the edges however you’d like (I like using my left hand’s thumb and pointer finger as a guide, then poking in with my right hand’s pointer finger, to form ruffles). Freeze for at least 15 minutes, until firm, or up to 2 days.
Parbake the crust: Place a rack in the lower third of the oven; heat to 400°F. Line the frozen crust with a square of parchment, fill with dried beans (or another pie weight), then set on a sheet pan. Bake, rotating halfway through, for about 40 minutes, until the crust is starting to brown along the edges and the bottom no longer looks raw and doughy.
Cook the spinach filling: Meanwhile, set a large skillet over medium to medium-low heat, then pour in the oil. When hot, add the onion and a small pinch of salt. Cook, stirring occasionally, for 5 to 7 minutes, until the onion is soft and mostly transparent. Stir in the garlic and cook for about 1 minute, until fragrant. Add as much spinach as will fit and stir, adding more spinach as soon as there’s space. Cook the spinach, stirring, until it’s just wilted, then season with another small pinch of salt. Transfer the spinach mixture to a plate to cool.
When the crust comes out of the oven, use a fork to whisk the egg white with a tiny pinch of salt until loose. Remove the parchment and pie weights, brush the inside of the crust with the salted egg white, then return to the oven to bake for another minute to set.
Remove the parbaked crust from the oven to cool and reduce the oven temperature to 325°F.
Make the custard: Pour the half-and-half into a large bowl. Place the flour in a medium bowl and add a splash (figure, 1 tablespoon or so) of the half-and-half. Stir with a fork or whisk until smooth. Repeat (splash, stir, splash, stir) until the flour mixture is smooth and pourable. Pour into the large bowl with the remaining half-and-half and stir to combine. Add the eggs, egg yolk, 1¼ teaspoons of the salt, and the pepper and whisk until smooth.
Fill the quiche: With the pie pan still on the sheet pan, sprinkle half of the cheese evenly over the bottom of the crust. Sprinkle the spinach mixture on top, spreading as much as possible (even scooching up the sides is fine), so that you can see the cheese and crust below. There should not be a packed-solid layer of mix-ins; the custard filling needs to be able to get through. Sprinkle the remaining cheese on top. Slowly and evenly pour in the custard—you should use all of it and the pan will be very full.
Bake the quiche for 55 to 65 minutes, until it no longer shimmies when shaken. (If the crust starts to brown too much mid-bake, you can carefully tent with foil.) Let cool until warm before serving. It’s also great at room temperature and cold.