To make the crust: In a large bowl, whisk together the flour(s) and salt. Work the butter into the flour until the largest pieces are the size of small peas. Stir in the water until a craggy mass forms., Knead the dough with your hands two or three times to form a ball, or use a bowl scraper to fold the dough over on itself until it comes together. Divide the dough into two pieces, with one slightly larger than the other., Pat each piece into a rectangle about 1/2” thick, then wrap and chill for 1 hour or as long as 2 days. Freeze for longer storage (defrost in the refrigerator overnight before rolling)., To make the filling: Place the potatoes in a medium saucepan and cover with cold, salted water. Bring to a boil and then reduce to a simmer, cooking for 7 to 10 minutes, until the potatoes are easily pierced with a paring knife. Remove from the heat, drain, and let cool., Wash the spinach, then cook the wet leaves in a large skillet over high heat until they just collapse. Drain in a colander, squeezing out as much liquid as possible; there should be about 1 cup spinach when you’re done., Preheat the oven to 375°F. Line a 10” x 15” x 1” jelly roll pan with parchment., To assemble: On a floured piece of parchment, roll the larger of the two dough pieces to an 18” x 13” rectangle. Use the parchment to help you transfer the dough to the prepared pan, flipping it over and peeling off the paper. Tuck the dough into the bottom of the pan and leave 3/4” overhanging the edges., Cut the potatoes in 3/8” slices and arrange in the pastry-lined pan. Roughly chop the spinach and distribute over the potatoes. Season with salt and pepper. Sprinkle the scallions and cheddar over all. Beat the eggs briefly, then slowly pour them over the filling in the pan., Roll the remaining dough into a 16” x 11” rectangle. Drape over the filling, trim the edges to line up with the edge of the pan, and fold the overhang from the bottom crust up and over to seal the pie. Pinch the edges and cut a few 1/2” slits in the lid to act as vents (too many or too big and the eggs will leak out). Beat the remaining egg yolk with water and brush it over the top crust., Bake the pie until the crust is golden and the filling is set, about 40 to 45 minutes. Remove from the oven and let cool on a rack for 10 minutes before cutting into squares., Store any leftovers, well wrapped, in the refrigerator for several days. Freeze for longer storage.