First, make the crust. In a large bowl, combine flour, sugar, and sea salt, and mix with a fork. Grate in cold butter, mixing in with fingertips until mixture is well combined. Drizzle in just enough water that mixture will hold together. Gather into a large disc and wrap tightly with plastic. Set in fridge; chill dough at least 30 minutes.
To prep filling, rinse peaches well and set out to dry. Halve peaches, remove pits, and cut into 1/2-inch thick slices. In a large bowl, toss peaches with sugar, ground tapioca, lemon zest and juice, and sea salt. Set aside while you roll out crust and prepare topping.
Preheat oven to 425 degrees F. Set a large rimmed baking sheet in oven to preheat as well. Butter a deep-sided 9 x 13-inch baking dish and set aside.
On a lightly floured surface, roll chilled disc of dough out into an 11 x 15-inch rectangle. Set into prepared baking dish, pressing dough all the way up the sides so that it’s even with the top of the dish. Set baking dish in the fridge to chill while you prepare the rest of the pie.
Remove pie shell from fridge and spoon in filling so that it mounds in the center. Cover top completely with crumb topping.
Set baking dish on preheated baking sheet and slide into oven. Bake at 425 degrees F for 15 minutes, then turn heat down to 375 degrees F, and bake an hour more. The pie is ready when crumb topping is a deep golden brown and filling is bubbling vigorously. Set pie on a wire rack to cool completely, 3 - 5 hours. This pie is best enjoyed the same day it’s baked, but as long as you’re OK with a slightly soggy bottom crust, it will keep just fine for 2 or 3 days.