In a medium mixing bowl add flour, salt, and sugar, stir to combine.
Then using a pastry blender (or a fork if you don't have) cut butter into flour mixture until mixture resembles coarse crumbs.
Add buttermilk to the center of the dough, stir until mixture starts to come together. If you need to add an extra teaspoon or two to bring the dough together, then do. I have used an additional tablespoon to make this crust.
Then start to knead the dough in the bowl. Once the dough has come together, turn dough out onto a lightly floured surface. Shape into a disk. Cover tightly with plastic wrap; chill in the refrigerator for 30 minutes.
Meanwhile, preheat the oven to 350°.
Roll out pie crust to 17 x 12 rectangle. Then roll dough into your 15 1/2 x 10 1/2- inch jelly-roll pan, pressing into bottom and up the sides. Crimp or fold edges as desired. Cover dough with a piece of parchment paper, letting ends extend over edges of pan.
Set another 15 1/2 x 10 1/2- inch jelly-roll pan on top of the parchment paper, the place in the oven for 20 minutes.
If you have pie weights set them on top of the jelly roll pan that is covering the dough. If you don't, use an oven-safe pot, this is what I did, or you can leave it just how it is, making sure to push down the top jelly roll pan.