Mix the flour, oats, brown sugar, cinnamon, and salt for the crumble. Cut in the cold butter until the mixture looks like damp sand with pea-sized lumps. Chill.

Whisk the sugar, cornstarch, lemon zest, and salt. Add the blueberries and vanilla, and fold until the berries are evenly coated.

Roll the dough into a 12-inch circle, fit it into a 9-inch pie plate, crimp the edges, and refrigerate for 15 minutes.

Preheat the oven to 375°F (190°C). Spoon the filling into the chilled crust, scatter the crumble on top, and bake on a rimmed baking sheet for 50 to 55 minutes, until bubbling and deep golden.

Cool on a wire rack for at least 3 hours before slicing. Serve at room temperature.