Cut butter into 1 Tablespoon sections. Place in freezer for 10 minutes. Place egg white into a 1/2 cup measuring cup or dish. Fill the rest of the way (1/2 cup fill line) with ice water.
Put flour into food processor. Add sea salt. Blend to mix. Remove cold butter from freezer and cut tablespoons in half, so they are in 1/2-tablespoon rectangles/sections. Add these to food processor and pulse to blend, until largest pieces are pea size.
Using only half the egg white-water mixture to start, pour over flour in a donut-shaped gully. Pulse. Add remaining egg white-water and process again, holding down Pulse button until mixture begins to come together: see photo~ about 10 seconds. Squeeze a bit of the dough to see if it holds together well. If not, add a bit more water, about 1/4 cup, and pulse again briefly.
Dump out contents of food processor onto a large square of parchment paper. (Reserve some if you've doubled the recipe to cut out shapes for the top.) Form into a circle with your hands, pressing and patting it together. The circle will be about 7" in diameter. Place second large square of parchment paper onto dough and begin rolling out with rolling pin, from the center, until you have an 11"-12" circle. (Ideally the center is rolled thinly, as this creates a flakier texture.) Peel off parchment from time to time and sprinkle cassava flour as you roll to keep it from sticking.
Transfer pie crust to pie plate by peeling off one side of parchment, placing pie plate upside down on pastry, sliding your hand under pastry that still has parchment, then flipping the whole thing over using both hands. If any breaks off you'll be able to piece it back together. Carefully peel remaining parchment paper off of top.
Allow crust to slide down into pie plate, fitting into concave shape. Fold over and flute/crimp top edges. If dough breaks during transfer, it easily pieces back together. Dust your fingers with cassava flour if edges are sticky. To bake the crust blind (without filling), prick all over with fork and bake in preheated oven for about 35 minutes. (To prevent the sides from collapsing in a couple of places, you can line the crust with parchment and pie weights for the first 15 minutes of baking; then empty and continue for remainder of baking time.) Otherwise fill and bake according to chosen recipe. If you wish, brush crimped edges of pie with egg yolk, to create a really golden outcome.
*If you use animal fat to make the crust, chill small dollops of the fat, as if you were chilling butter: first measure the scoopable fat into a measuring cup; then place small dollops of the fat onto a parchment lined cookie sheet. Place fat into freezer for 10 minutes. Do not, however, chill tallow, as it will harden too much. Both duck fat and lard work well. Lard is best when it's leaf lard, the fat around the kidneys — perfect for making pastry dough. I often point clients to etsy for this product if you don't know a local pastured-pig farmer: here.**To make an egg-free version of this recipe, simply omit the egg white and use the ice water in its place, about 1/2 cup. The crust will not be quite as flaky under the filling; but the edges will be very flaky. You can also do one of two things, if your diet allows:
Add a bit of xanthan gum if your diet allows (1/4 teaspoon blended with the flour), if you wish, to increase the flakiness. Xanthan gum is not needed to hold the pie crust together. However, I find that it affects the texture of the baked crust, creating a flakier outcome. Here is a non-GM product.
Or you can use 1 tablespoon Psyllium Husk Powder here, and blend it with the flour. (Sorry these are not AIP options!)