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1
Make the crust: Combine the flours, sugar, salt, and baking powder in the bowl of a stand mixer fitted with a paddle attachment. Mix on low just to combine. Add the cream cheese and mix on low until them cream cheese is mostly broken-down. Add the butter and mix on low 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). Combine the water and vinegar in a small glass and, with the machine running, pour in. 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 in plastic 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.)
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2
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. Use scissors to trim the dough to an even 1-inch overhang. Fold the edge under itself. Use your fingers to 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 crimping with my right hand's pointer finger, to form ruffles). Stick in the freezer for at least 15 minutes or up to 2 days.
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3
Parbake the crust: Heat the oven to 400 degrees. Line the frozen crust with a square of parchment, fill with dried beans (or another pie weight), then set on a sheet pan. Bake on the lower rack for about 40 minutes, rotating halfway through, until the crust is starting to brown along the edges and the bottom no longer looks raw and doughy. While the crust is baking, use a fork to whisk the egg white with a pinch of salt until it's very loose. When the crust is done, remove the parchment and beans and prick the bottom and sides of the crust a few times with a fork. Brush the inside of the crust with the salted egg white, then return to the oven to bake for another minute to set.
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4
Remove the parbaked crust from the oven to cool, then lower the oven to 325u00b0F and turn your attention to the custard.
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5
Make the custard: Combine the cream and milk in a bowl. Add the flour to another, smaller bowl, and add a splash (figure, 1 tablespoon or so) of the liquid. Stir with a fork or whisk until smooth. Repeat this (splash, stir, splash, stir) until the flour mixture is completely smooth and pourable. Pour into the bigger bowl of cream and milk and stir. Add the eggs, egg yolk, salt, and pepper, and whisk until smooth.
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6
Fill the quiche: Sprinkle half of the cheese evenly over the bottom of the crust. Sprinkle the mix-ins on top, spreading them out 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, taking it as high to the brim as possible. It should be on the verge of overflowing. If you have any leftover custard, bake it in a ramekin or microwave it as a snack.
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7
Bake the quiche for 55 to 65 minutes, or until it no longer shimmies when shook. (If the crust starts to brown too much mid-bake, you can carefully tent it with foil.) Let cool until warm before serving. It's also great at room temperature and cold.
