Preheat the oven.
Set the oven to 375°F so it’s ready once your crisp is assembled.
Make the filling.
Whisk together applesauce, cornstarch, cinnamon, and brown sugar until completely smooth and the cornstarch is fully dissolved.
Add sliced apples.
Stir the apples into the applesauce mixture to evenly coat every slice.
Spread into the baking dish.
Pour the mixture into a greased 9×13 pan and smooth the top.
Make the crumble topping.
Combine oats, flour, cinnamon, brown sugar, and granulated sugar in a bowl. Add cold, cubed butter and work it in with your fingers until the mixture looks evenly moistened and crumbly.
Top and bake.
Sprinkle the oat topping evenly over the filling. Bake for about 40 minutes, or until the topping is golden and the edges are bubbling.
Rest before serving.
Let the crisp cool for about 20 minutes to allow the filling to thicken. Serve warm with whipped cream or a big scoop of vanilla ice cream.