Preheat your oven to 180°C (350°F). Lightly grease an 8x8 inch (20x20 cm) baking dish or equivalent oven-proof pie plate.

Prepare the fruit filling: In a large bowl, combine the chopped rhubarb, apples, caster sugar, cornflour, and vanilla extract. Toss gently until the fruit is evenly coated. Transfer the mixture to the prepared baking dish and spread it into an even layer.

Make the crumble topping: In a separate bowl, combine the flour, rolled oats, and brown sugar. Add the cold, cubed butter. Using your fingertips, rub the butter into the dry ingredients until the mixture resembles coarse breadcrumbs with some pea-sized clumps remaining.

Sprinkle the crumble topping evenly over the fruit layer in the baking dish, ensuring all the fruit is covered. Do not press the topping down.

Bake for 35 to 40 minutes, or until the topping is golden brown and crisp, and the fruit filling underneath is bubbling thickly.

Remove from the oven and let it cool for at least 10 minutes before serving to allow the filling to set slightly. Serve warm with heavy cream, custard, or vanilla ice cream.