For the filling, mix together the blueberries, white sugar, flour, cornstarch, lemon juice, and lemon zest.
For the crumble, combine the light brown sugar, flour, and cinnamon. Pour in the melted butter.
Combine the crumble mixture until everything is wet and forming clumps. Mix in the oats.
Bake this at 400 for 25 minutes. Then lower the temp to 375 and bake for another 30 minutes. 10 minutes before it’s finished, put a sheet of aluminum foil on top to save the crumble from browning any further.
Crumble straight out of the oven is SUPER hot, let cool before eating! Enjoy with a scoop of vanilla ice cream or some whipped cream :)