This is hands-down the Best Apple Pie ever. The filling is loaded with just the right amount of apples and sweetness. The flaky butter crust is unbelievably delicious. Dough can be prepared ahead of time.
Prepare the Dough: In a large bowl, combine flour, salt, and sugar. Using pastry cutter or clean hands, cut in butter until dough mixture resembles coarse crumbs, with small pea-sized butter chunks in the dough.
Add 1/4 cup of the ice-cold water, and gently knead until dough comes together - if needed, add 1 TB at a time of additional cold water as needed, just until mixture is pliable and when you pinch the dough, it holds together - not crumbly nor too sticky. Work dough as sparingly as possible. Small chunks of butter should still be visible in the dough.
Divide into 2 even dough balls. Press them into discs with smooth edges, wrap them in plastic, and chill until ready to use (preferably at least an hour, up to overnight.)
Take dough discs out of fridge; if it's too firm to roll, let it rest at room temperature 5-10 minutes or until it's malleable but still cold. On a lightly floured surface, roll first disc to fit bottom and sides of a 9" pie pan, with 1/2" overhang evenly beyond pan edges, trimming as needed. Roll the other disc to fit over top of pie pan with 1" overhang all around edges. Cover with cling wrap and place both crusts in fridge.
Preheat oven to 425F, with rack on lower middle position. Peel and cut apples into 1/4" slices. Melt butter in a saucepan over medium-low heat. Stir in flour. Add juice, both sugars, vanilla, cinnamon, and nutmeg. Bring to a low boil stir well, and remove from heat.
In a large bowl, combine apple slices with most of the warm butter-sugar sauce, reserving about 1/4 cup for brushing over top crust. Gently transfer apple pie filling into the prepared pie pan, forming a mound. Place top crust over filling and tuck excess edges of top crust under excess edges of bottom crust. Pinch or use fork to crimp very tightly all around edges to ensure a tight seal. Cut several slits in top crust.
If needed, add just a bit of water to reserved 1/4 cup butter-sugar sauce, to thin it out just a bit. Brush evenly over entire top crust. Sprinkle coarse sugar generously in an even layer over crust. Sprinkle a pinch of cinnamon on top, if desired.
Place pie on a baking sheet and bake 15 minutes. Then reduce oven to 350F and bake about 50-55 minutes, or until top crust is golden brown and crisp. Check halfway through bake time - if edges are browning too quickly, use a pie crust shield or tent loosely with foil.
Cool completely before serving. Cooling on counter overnight is best. Enjoy...it is totally worth your work!