A flaky, buttery pie crust cradles a jammy, sliceable blueberry filling brightened with fresh lemon, all crowned with a crisp brown sugar oat crumble.
Prepare the pie dough: Make and chill a single-crust pie dough for at least 2 hours until very cold.
Make the crumble topping: In a bowl, combine brown sugar, all-purpose flour, cinnamon, and salt. Cut in very cold, cubed butter with a pastry cutter or 2 forks until crumbles form. Stir in old-fashioned oats. Refrigerate the topping to keep it cold.
Make the filling: Rinse and pat blueberries dry. Lightly mash a few berries. Toss berries with granulated sugar, all-purpose flour, cornstarch, lemon zest, and lemon juice until the mixture looks shiny and juicy (not dry or powdery). Refrigerate while preparing the crust.
Assemble the pie: On a floured surface, roll the chilled dough into a 12-inch circle. Fit into a 9-inch deep-dish glass pie plate. Flute or crimp edges. Brush crust edges with egg wash (egg beaten with a little milk).
Fill and top: Add the blueberry filling to the crust and mound the cold crumble topping evenly over the berries.
Bake initial stage: Bake at 400°F (204°C) for 25 minutes to set the crust and start thickening the filling.
Finish baking: Reduce oven temperature to 375°F (190°C) and continue baking until the juices are actively bubbling around the edges and the crumble is golden; tent the pie or add a crust shield if browning too quickly. For accuracy, bake until the center reaches about 200°F (93°C).
Cool completely: Let the pie cool for several hours (overnight is best) to allow the filling to fully set before slicing. Serve plain or with vanilla ice cream and whipped cream.