1. Peel, core and roughly chop the apples. Stone and quarter the plums. Place both in a saucepan over a medium heat along with the creme de cassis, simmering for about 15 minutes until tender. Mix the teaspoon of cornflour with a little cold water (about 2tbs), making a runny paste, and stir into the cooled fruit. Tip into an oven-proof serving dish.
pre-heat the oven to 180 degrees C
2. Mix together the flour and sugar in a small bowl, rubbing in the butter with your finger tips until you reach the consistency of fine breadcrumbs, though do not worry if the odd piece of chunky butter remains.
3. Sprinkle the crumble mix over the fruit and bake in the pre-heated oven for about 20 minutes or until the topping is golden and firm to touch.