Add the blueberries, cornstarch, light brown sugar, granulated sugar, lemon zest, lemon juice, and vanilla to the dish. Stir everything together until well combined.

In a medium bowl, prepare the crumble topping by mixing flour, oats, cinnamon, baking soda, sugars, and salt.

Add the melted butter, vanilla, and egg, then stir until the mixture forms a dough similar to oatmeal cookie dough.

Using your hands, gently distribute the cookie dough over the blueberry mixture, making sure not to press it down into the berries.

Bake for 25–30 minutes, or until the topping turns golden brown.

Remove from the oven and let it cool for about 30 minutes before serving.