Core and chop apple into bite-sized pieces. I keep the skin on, but you are welcome to remove it if desired.
Spray slow cooker with oil or grease with butter on the bottom and sides. Combine all ingredients except the pecans or walnuts into slow cooker and stir to combine.
Cover and cook on low for 2 to 3 hours or until all or most of the liquid is absorbed (varies by your oatmeal preference) and apples are soft. You can cook for a shorter time if you like a bit of chewiness to your oatmeal, or longer if you prefer them very soft.
Before serving, stir in chopped nuts of choice and serve with an optional drizzle of honey or maple and a splash of milk or cream.
Overnight Cooking: If you want to cook overnight, add an extra 1/2 cup of milk, cover and set to 'Keep Warm' setting for about 8 to 10 hours. Add an extra splash of milk or cream if a bit dry when ready.