Preheat your oven to 375°F (190°C). Lightly grease an 8x8-inch (or similar) baking dish.
Make the Filling
In a large bowl, combine blueberries, sugar, cornstarch, lemon juice, vanilla, and salt. Stir gently until the berries are evenly coated. Pour the mixture into your prepared baking dish.
Mix the Crumble Topping
In another bowl, mix oats, flour, brown sugar, cinnamon, and salt. Drizzle in the melted butter and stir until the mixture becomes crumbly and moist.
Assemble and Bake
Sprinkle the crumble topping evenly over the blueberries. Bake for 35–40 minutes, or until the topping is golden brown and the filling is bubbling.
Cool Slightly and Serve
Let the crumble cool for about 10 minutes before serving. Enjoy warm with a scoop of vanilla ice cream, whipped cream, or just as is!