Preheat your oven to 375°F (190°C). Lightly grease a 9-inch baking dish. This prevents sticking and makes serving much easier after baking.

Place blueberries in a large bowl. Add sugar, cornstarch, lemon juice, and vanilla extract. Stir gently until the berries are coated evenly and everything is well combined.

Pour the blueberry mixture into the prepared baking dish. Spread it evenly with a spoon. This helps the berries cook at the same rate.

In another bowl, combine flour, oats, brown sugar, cinnamon, and salt. Stir everything together until the ingredients are evenly mixed throughout.

Add the cold butter cubes to the dry mixture. Use a fork or pastry cutter. Mix until you see small crumbs forming throughout the mixture.

Sprinkle the crumble mixture evenly over the blueberry filling. Make sure the berries are covered well for the perfect balance of fruit and topping.

Place the dish into the preheated oven. Bake for about 40 minutes. The topping should turn golden brown and the filling should bubble.

Remove the dessert from the oven carefully. Let it cool for at least 15 minutes. This helps the filling thicken before serving.

Serve warm on its own or with vanilla ice cream. The warm blueberries and crunchy topping create a delicious combination everyone loves.