Preheat the oven to 400ºF/200ªC
Toss the cherries with the cornstarch and sugar in a large bowl.
Place into an oven proof baking pan. I used a 9x13 inch baking dish - but it can be slightly larger or smaller since crumble is rustic.
Mix the flour and sugar together in a mixing bowl.
Add the cubed butter and rub it into the flour mixture with your fingertips until it resembles coarse breadcrumbs.
Sprinkle the crumble topping mixture over the fruit leaving a small border at the edge so you can see a little of the cherries.
Place on a baking sheet and bake for 35-40 minutes or until hot and the crumble is golden brown.
Serve with a scoop of ice cream or even better - custard!
