3 3/4 cups all-purpose flour feel free to replace up to half with whole-wheat
1 1/2 teaspoons table salt
3 sticks unsalted butter very cold
3/4 cup water very cold
1 pound yukon gold potatoes peeled if desired, cut into 1/2-inch slices
10 ounces spinach baby, “grown-up,” or frozen
1 cup sharp cheddar cheese coarsely grated
4 scallions thinly sliced
11 large eggs
1 egg white large; you'll use the yolk in a different step
1 teaspoon kosher or coarse sea salt plus more to taste
Freshly ground black pepper
1 large egg yolk leftover from egg white
1 teaspoon water
Equipment
whisk
medium saucepan
10x15x1-inch baking sheet
oven
drainer
pastry blender
plastic wrap
rubber spatula
large bowl
floured surface
Directions
Whisk together flour, and salt in the bottom of a large, wide-ish bowl. Using a pastry blender, two forks, or your fingertips, work the butter into the flour until the biggest pieces of butter are the size of tiny peas. Gently stir in the water with a rubber spatula, mixing it until a craggy mass forms. Get your hands in the bowl and knead it just two or three times to form a ball. Divide dough roughly in half. Wrap each half in plastic wrap and flatten a bit, like a disc. Chill in fridge for at least an hour.
Heat oven to 375 degrees F. Line bottom of 10x15x1-inch baking sheet or jellyroll pan with parchment paper.
Place potatoes in a medium saucepan and cover with cold water. Bring to a boil, then reduce to a simmer, cooking for 7 to 10 minutes, until potatoes are tender but not falling apart. Drain.
Wash spinach but no need to dry it. Place wet spinach in hot skillet and cook until it just wilts. Drain in a colander, pressing or squeezing out as much liquid as possible. Roughly chop the squeezed spinach if leaves were large.
On a lightly floured surface, roll one of your dough halves into an 18-by-13-inch rectangle. Transfer to your prepared baking sheet and gently drape some of the overhang in so that the dough fills out the inner edges and corners. Trim this to 3/4-inch overhang.
Layer vegetables, including scallions, evenly over bottom pie crust. Sprinkle cheese on top. Beat 11 whole eggs and 1 egg white lightly and pour over vegetables. Sprinkle with salt and many grinds of black pepper.
Roll the second of your dough halves into a 16-by-11-inch rectangle. Drape over filling and fold the bottom crust’s overhang over the edges sealing them together. Cut only a couple tiny slits in the lid to act as vents. Beat remaining egg yolk with 1 teaspoon water and brush over lid.
Bake pie until crust is golden and filling is set, about 40 to 45 minutes. Transfer to a wire rack and cool a bit before cutting into squares.