Place all ingredients in a slow cooker and stir until well combined.
Cook overnight on Low for 8 hours.
In the morning, stir oatmeal with a fork to loosen the steel cut oats and make sure everything is fully incorporated.
Serve warm with banana slices, chopped walnuts or a pinch of brown sugar if you wish.
Video
Notes
For easier clean-up and to avoid the slight "crusty" residue inside the slow cooker, place all ingredients in a glass bowl that fits into the slow cooker then add enough water to fill the lower half of the slow cooker. Cover and cook on low for 6 to 8 hours.