Prepare the fruit: Heat the oven to 350°F. Grease a 9-x-13-inch baking dish with butter and set aside. Core and thinly slice the apples. Toss the fruit with lemon juice, cornstarch, sugar, and cinnamon. Dump into the prepared baking dish.

Prepare the oat topping: Thoroughly mix together all of the dry ingredients in a large bowl with a fork. Add the butter and pinch it with your fingers until the topping is in smallish clumps. Scatter the topping over the fruit.

Bake and serve: Bake until juices are bubbling around the edges the topping is golden, 55 minutes to 1 hour. Remove from the oven and cool on a wire rack. Serve warm with whipped cream or vanilla ice cream.