Preheat the oven to 350°F. Spray a 9x13-inch baking pan with nonstick cooking spray.
In a large mixing bowl, combine the peeled sliced apples, applesauce, light brown sugar, lemon juice, cornstarch, and ground cinnamon. Stir until evenly mixed together and then transfer the mixture to the prepared baking dish and spread it into an even layer.
6 Granny Smith apples, 2 cups applesauce, ½ cup packed light brown sugar, 1 tablespoon fresh squeezed lemon juice, 1 tablespoon cornstarch, 1 teaspoon ground cinnamon
In a separate bowl, whisk together the old-fashioned oats, flour, light brown sugar, and cinnamon.
1 cup whole old-fashioned oats, ¾ cup all-purpose flour, ½ cup packed light brown sugar, 1 teaspoon ground cinnamon
Stir in the melted butter until the mixture is crumbly and resembles wet sand with large chunks.
½ cup (1 stick) salted butter
Sprinkle the oat crumble topping mixture evenly over top of the apples inside the baking pan.
Bake for 50-60 minutes until golden brown and bubbly.
Serve the applesauce crisp warm with a scoop of vanilla ice cream.