Whisk together egg, milk and vanilla. Pour over bread and let soak for several minutes.
Meanwhile, make the topping by mixing the butter, brown sugar and pecans in a small bowl. Set aside.
Spray the inside of the slow cooker with cooking spray and add the soaked bread.
Top with apple slices, arranging evenly.
Crumble the topping over the apples and bread mixture.
Cover and cook on HIGH for 2.5 hours or on LOW for 4.5 hours.
Serve warm with butter and syrup, if desired. Feeds 6-8.