Preheat your oven to 375°F (190°C).
In a mixing bowl, toss the blueberries with lemon juice, granulated sugar, and cornstarch. Pour them into a greased baking dish.
In another bowl, mix oats, flour, brown sugar, cinnamon, and salt. Drizzle in the melted butter and stir until everything looks crumbly and glorious.
Sprinkle the topping evenly over the berries.
Bake for 35 to 40 minutes or until the top is golden brown and the berries are bubbling around the edges.
Let it cool a bit before digging in—though no judgment if you grab a spoon straight from the dish!