For the pie crust, place the flour, salt, and sugar in a medium bowl and whisk to combine. Add the cubed butter and toss to coat with flour mixture. Using your thumb and forefinger gently flatten each cube of butter.
Add the apple cider vinegar and 3 Tbsp of ice water to the flour-butter mixture. Toss with a fork. Continue adding 2 Tbsp of water at a time and tossing with a fork until the dough starts to look shaggy. Turn the dough out onto a floured work surface. Resist the urge to knead the dough.
Using your hands, pull the shaggy dough into a round then lightly flatten. Fold the dough in half, flatten, rotate 90°, and repeat this process three more times. Cut the dough in half and form into a round. Flatten to about 1-inch thickness. Wrap each round in plastic wrap and place in the fridge for 1 hour.
For the quiche, preheat oven to 325°F with a rack positioned in the center. Roll out one pie round to fill a 9-inch pie plate. Reserve the other pie crust for another pie, or freeze for later. Partially bake the pie crust for 10-12 minutes, until lightly golden. Set aside to cool.
Place eggs in a blender and blend on high speed until frothy, about 30 seconds. Add half-and-half, salt, black pepper, and nutmeg to the egg mixture. Blend on medium until the mixture is smooth, about 10 seconds. Add the Gruyère, stirring to combine.
Arrange the greens in the bottom of the pie crust. Pour the custard over the greens and gently wiggle to remove any air bubbles. Bake quiche until edges are set and center is still slightly wobbly, 55-70 minutes. Remove from oven and let cool at least 2 hours before slicing.
Can also be made 1-2 days in advance and stored in the fridge, wrapped tightly until ready to serve.