Grind the oats in the food processor until fine. Add the whole wheat pastry flour, buttermilk powder, baking powder, and salt. Pulse about 5 times to mix. Add the butter cubes and pulse until the cubes are broken into pea sized pieces and evenly distributed, about 10 times. Continue to pulse and drizzle in the orange juice, then add the ice water 1 tablespoon at a time until the dough starts to clump together. It should not be too wet or sticky.
Scrape the dough onto a piece of plastic wrap and shape the dough into a flat disk. Wrap the plastic tightly around the dough and chill in the refrigerator until the grain fully absorbs the liquid and softens, overnight or up to 3 days. Remove the dough from the fridge and allow to soften, about 15 to 30 minutes.
Preheat the oven to 375° F/190° C. Sprinkle flour on a work board and roll the dough into a 12-inch circle. Transfer dough to the pie plate and trim the edges. Make a standing crimp tall enough to hold the filling and crumb topping. Cover with plastic wrap and chill while preparing the filling.
If using thawed peaches, drain and reserve just 3 tablespoons of the juice. Cook the peaches in the medium saucepan with the sugar and the salt until the sugar dissolves and the peaches release their juice. Mash the peaches into large chunks, and drain off some of the liquid if the peaches are swimming. The blueberries will generate liquid when baking, so only leave about 1 to 2 tablespoons of the peach liquid.
Stir in the nutmeg, cinnamon, lemon juice, vanilla extract, and blueberries. Whisk together the cold water and cornstarch and stir into the peach filling. Remove from heat and allow to cool.
Grind ⅔ cup of oats in the food processor until fine, then add the brown sugar, cinnamon, and salt and pulse a few times until fully mixed. Pour into a medium bowl and whisk in the remaining whole oats.
Assemble & Bake
Remove the chilled crust from the refrigerator and add the peach and blueberry filling. Spoon all of the crumb topping over the filling and gently lay a piece of foil on top to prevent the topping from burning. Place pie plate in the oven on the foil-lined baking sheet.
Bake until the crust and topping are golden brown and the filling is bubbling, about 50 minutes to 1 hour. Remove the pie from the oven and allow to cool at room temperature before cutting, for at least 4 hours.