Coat the inside of a Crockpot with nonstick cooking spray. Add the steel-cut oats, water and salt. Stir to combine.
Nestle the peeled sweet potato halves on top of the oat mixture with the cut sides facing up. Do not stir them into the oats.
Sprinkle the brown sugar, cinnamon and nutmeg over the exposed sweet potato flesh. Divide the butter between the halves, placing it directly on top.
Cover and cook on Low for approximately 7 hours, until the oats are cooked and the sweet potato mashes easily. If the sweet potato is still firm in the center, cover and continue cooking until tender.
Use a potato masher to mash the sweet potato directly into the oatmeal. Add the vanilla and stir thoroughly until evenly combined.
Let stand for 5–10 minutes. Spoon into bowls and serve with your choice of toppings.
