Whisk the crust flour, sugar, and salt in a large bowl.
Cut in the cold diced butter until the mixture looks like coarse crumbs with pea-sized bits.
Sprinkle in ice water, tossing just until a shaggy dough holds together without being sticky.
Pat into a rectangle, wrap, and chill 20 minutes for easier rolling.
Preheat oven to 375°F. Line a 10x15-inch rimmed baking pan with parchment.
Roll the dough into a large thin rectangle, about 1/8 inch thick.
Fit dough into the pan, trim excess, and crimp edges. Chill 10 minutes to firm up.
Dock the crust with a fork, line with parchment, add pie weights, and bake 12 minutes. Remove weights.
For the filling, whisk sugar, flour, salt, and nutmeg in a bowl until evenly combined.
Whisk in beaten eggs, buttermilk, melted butter, vanilla, lemon zest, and juice until smooth and creamy.
Pour the filling into the warm crust and gently tap the pan to release bubbles.
Bake 30 to 35 minutes, until the center has a slight jiggle and edges are set and golden.
Cool completely on a rack. Chill 1 hour for clean slices, then cut into squares and serve.