Prepare the Crust: Start by placing your pie crust into a 9 inch pie dish. Press it gently into the bottom and sides so it fits snugly. Trim any excess dough around the edges and crimp if desired. Place the crust in the fridge while you prepare the filling to keep it firm
Make the Blueberry Filling: Add blueberries, sugar, cornstarch, lemon juice, lemon zest, cinnamon, salt, and vanilla to a large bowl. Stir gently until the berries look evenly coated. The mixture will look loose at first, which is normal. Let it sit for a few minutes so the sugar starts drawing out the juices.
Fill the Pie: Pour the blueberry mixture into the chilled crust. Spread it out evenly so the filling sits level. Tap the pie dish lightly on the counter to help settle the berries and remove air pockets.
Prepare the Crumble Topping: In a bowl, mix flour, brown sugar, cinnamon, and salt. Add the cold butter cubes and use your fingers or a pastry cutter to work the butter into the mixture. Keep mixing until large crumbs form and no dry flour remains. The texture should feel crumbly and slightly clumpy.
Add the Crumble: Sprinkle the crumble topping evenly over the blueberry filling. Cover the surface fully, then gently press the crumbs so they stick together during baking. If using nuts or coarse sugar, scatter them on top now.
Bake the Pie: Preheat the oven to 375°F. Place the pie on a baking sheet to catch any drips. Bake for 45 to 55 minutes until the filling bubbles around the edges and the topping turns golden brown. If the topping browns too quickly, cover loosely with foil.
Cool Before Slicing: Remove the pie from the oven and let it cool for at least 2 hours. This cooling time helps the filling set so slices hold their shape. Cutting too soon can cause the filling to run.
