1. Bring milk and 1 1/2 cups water to a boil in a saucepan over medium-high heat, stirring frequently.
2. Coat a 3 1/2-quart electric slow cooker with cooking spray. Place hot milk mixture, apple, and next 5 ingredients (through salt) in slow cooker; stir well. Cover and cook on LOW for 7 hours or until oats are tender.
3. Spoon oatmeal into bowls; top with maple syrup and hazelnuts.