Wash the blueberries thoroughly.
In a bowl, mix blueberries with a small amount of sugar and lemon juice.
In another bowl, combine cake flour, almond flour, oatmeal, and butter, mixing by hand to make a crumble dough.
Spread the blueberry mixture in an oven-safe dish and evenly sprinkle the crumble dough on top.
Bake in a preheated oven at 180°C (356°F) for 25 minutes.