Coat the slow-cooker with cooking spray or grease with butter.
Put the fruit in the cooker, cover, and cook on HIGH for 30 minutes.
Meanwhile, make the topping: in a bowl, combine the rolled oats, flour, brown sugar, baking powder, cinnamon, nutmeg, and salt.
Cut in the butter using a pastry blender or your fingertips; set aside.
After the 30 minutes, spread the topping evenly over the fruit, leaving a ½-inch border without topping to prevent burning.
Cover and decrease heat to LOW; cook 2 ½ to 3 hours until the fruit is tender.
Test by sticking a knife into the center of the crumble; when the knife passes through the fruit with little resistance, the crumble is done.
Uncover and let cool for 10 minutes before serving; if desired, top with ice cream or whipped cream.