Coat slow cooker with butter-flavored cooking spray or grease with butter.
Make the fruit mixture: put the fruit in the cooker; sprinkle with cinnamon, sugar, and cornstarch; toss to coat the fruit.
Cover and cook on HIGH for 30 minutes.
Make the topping: in a food processor, combine the flour, rolled oats, brown sugar, and cinnamon; add in butter and pulse to make coarse crumbs.
After the 30 minutes, spread the topping evenly over the fruit, leaving a ½-inch border without topping to prevent burning.
Cover, decrease heat to LOW, and cook 2 ½ to 3 ½ hours until the fruit is tender.
Test by sticking a knife into the center of the crumble, when it passes through the fruit with little resistance, the crumble is done.
Uncover and let cool for 10 minutes before serving; serve with ice cream or whipped cream if desired.