In a large bowl, whisk the 2 ½ cups whole wheat flour, 1 teaspoon granulated sugar, and ½ teaspoon fine sea salt. Cut in the ¾ cup unsalted butter, until the mixture resembles coarse crumbs. Add the 6-7 tablespoons ice water 1 tablespoon at a time, and mix it in with a fork until the pastry holds together lightly when pressed into a ball.
Divide into two balls, wrap them in plastic, flatten them into discs, and let chill in the fridge for at least 30 minutes.
When ready to roll out the dough, remove it from the fridge and lightly dust a clean surface with oat flour. Roll out the dough and use it in your recipe.
Notes
Keep the butter cold. Something that really helps create an ultra-flaky pie dough is to keep the butter very cold. This will create streaks and clumps of butter in the dough - and that's a good thing! When you bake the crust, the butter will melt in the oven, creating gaps in the dough. Steam also releases during the baking process, making those lovely flaky layers. Keep the butter in the fridge until just before using, or even keep it in the freezer so it's ultra cold.
Add ice water slowly. You want just enough cold water to bring the dough together, but you don't want to make it too wet and goopy. You'll get the hang of this the more you make pie dough, but a good rule to follow is to continue to slowly add water 1 tablespoon at a time until you don't see any dry bits of flour. The dough should hold together when you squeeze it into a ball.
Hydrate the dough. Whole wheat flour, like gluten-free flour, benefits from a period of rest to help it hydrate. Leave at least 30 minutes for the dough to chill in the fridge, and preferably even more if you can. You can keep the dough in the fridge for up to 3 days, or freeze it for longer. The colder the dough, the easier it will be to roll out. However, you may need to let the dough rest on the counter for a few minutes if it's very chilled.
Make it vegan. You can easily make this recipe vegan by using your favorite non-dairy butter for the butter. Or, try cold coconut oil or olive oil.
Follow your own recipe. You may or may not need to blind-bake (cook the pie briefly at a higher temperature) depending on the recipe you're using. For fruit pies, such as apple pie, you won't need to blind-bake. But for a custard-based pie, such as pumpkin pie, you may want to blind-bake before adding the filling. It really just depends on your recipe.