Preheat the oven to 350ºF. Grease a 9x9 pan with butter or a light layer of baking spray.
In a medium bowl, use a fork to combine the oats, brown sugar, flour, cinnamon, baking soda, and baking powder. Once it's evenly mixed, pour the melted butter on top and gently mix again, just until the mixture starts to come together. Don’t overwork it, you want nice big clumps. Place the bowl in the refrigerator for at least 10 minutes while you prepare the filling. 1 cup old-fashioned rolled oats, 1 cup brown sugar, ½ cup all-purpose flour, ¾ teaspoon cinnamon, ¼ teaspoon baking soda, ¼ teaspoon baking powder, 8 tabelspoons butter
Add the apples to a large bowl with the granulated sugar and cinnamon. Stir well until evenly combined, then pour the mixture into the prepared pan.
6 cups (about 5-7 apples, or 1½ pounds after cutting) Granny Smith apples, ⅓ cup granulated sugar, ½ teaspoon cinnamon
In a small bowl or liquid measuring cup, mix together the cornstarch and apple juice (or water or rum) until the cornstarch is dissolved. Pour that mixture evenly across the top of the apples.
2 tablespoons apple juice, 1 tablespoon cornstarch
Take the topping out of the refrigerator and sprinkle it evenly across the apples. If the topping isn't "chunky" enough, squeeze it gently in your hands as you sprinkle to form more clumps.
Bake for 40-45 minutes, until the apples are fork tender and the topping is browned. Allow it to cool for 5-10 minutes in the pan, then serve warm with vanilla ice cream.