Preheat oven to 400.
In the bowl of a food processor, combine flour, Essence, salt, and butter and process until mixture resembles coarse crumbs. While the motor is running, add water in increments until dough comes together to form a ball. Transfer dough to a lightly floured surface and shape into a flat disc. Refrigerate overnight (or at least one hour).
Roll out pie crust on a lightly floured surface to fit a deep 10-inch pie pan. Place crust in pan and crimp edges. Refrigerate at least 30 minutes, then line with parchment paper. Fill with pie weights and blind bake for 12 to 15 minutes, or until lightly golden around edges. Remove parchment paper and pie weights and return crust to oven for 3 to 5 minutes. Remove from oven, brush surface of crust with a light coating of egg white, and allow to cool.
Set a box grater on a clean kitchen towel. Grate potatoes on the large holes of the grater onto the towel. Gather the ends of the towel up around the grated potatoes, hold over the sink, and squeeze to release excess moisture from potatoes. Transfer potatoes to another clean towel and squeeze again. (You may need to do this in batches.) Reduce oven temperature to 375.
Add bacon to a small skillet and set over medium heat. Cook until fat is rendered and bacon is crispy, about 10 minutes. Using a slotted spoon, transfer bacon to a paper-towel-lined plate and reserve. Discard all but 2 tablespoons bacon fat from pan.
Add the leek, 1/2 teaspoon salt, and 1/4 teaspoon white pepper. Cook, stirring as needed, until leek is soft, 6 to 7 minutes. Remove from heat.
In a large bowl, whisk the eggs with the sour cream, milk, cream, thyme, green onion, grated cheddar, grated potatoes, and remaining 1 teaspoon salt, 1/2 teaspoon white pepper, and 1/4 teaspoon cayenne until smooth. Add bacon and leeks and stir to combine. Carefully ladle filling into crust.
Bake for 25 to 30 minutes. Reduce oven temperature to 325 and cook for an additional 15 minutes, or until quiche is nearly set in the center. (It will continue to set as it cools.) Remove quiche from oven and set aside at least 20 minutes before slicing and serving.