Peel, slice and core 3-4 pounds of Honeycrisp or Granny Smith apples.
In a large bowl, combine the apple slices, sugar, flour, salt, cinnamon and vanilla. Mix well.
Spread the apples evenly in a 6-quart slow cooker (or a 9×13” casserole slow cooker).
Using the same bowl that you mixed the apples in, whisk together the oats, brown sugar, flour, soda and baking powder.
Using a pastry blender, cut in the cold butter until the mixture is crumbly.
Sprinkle the buttery oat crumbs on top of the apple mixture.
Cover and cook on low for 4-5 hours until the apples are tender.
If you'd like a crisper top to the oat streusel, see our recipe article for 3 ways to get that crisp.