Preheat your oven to 180°C. For the topping, mix together your dry ingredients in a large bowl, then add the butter. Use your hands to mix together until the mixture reaches a crumb consistency.
In a separate large bowl mix all of your filling ingredients, adding the cornflour last by scattering it over the fruit, then stir until cornflour is no longer visible.
Transfer your filling to an oven-safe dish measuring approx. 16 x 16 cm (roughly 6 x 6″). Even out the top of the filling using a flat spatula.
Gently sprinkle the topping on top, ensuring it’s spread evenly over the fruit filling.
Bake at 180°C for 35-40 minutes. If the top is starting to brown too much, cover with aluminium foil for the rest of your bake.