Crust
3 3/4 (470 grams) cups all-purpose flour (feel free to replace up to half with whole-wheat)
1 1/2 teaspoons table salt
3 sticks (340 grams) unsalted butter, very cold
3/4 cup very cold water

Filling
1 pound yukon gold potatoes, peeled if desired, cut into 1/2-inch slices
10 ounces spinach (baby, “grown-up,” or frozen)
1 cup coarsely grated sharp cheddar cheese
4 scallions, thinly sliced
11 large eggs + 1 large egg white (you’ll use the yolk in a minute)
1 teaspoon kosher or coarse sea salt, plus more to taste
Freshly ground black pepper

To finish
1 large egg yolk (leftover from filling)
1 teaspoon water