Spray the inside of the slow cooker with cooking spray. Add all of the steel cut oat ingredients except for the brown sugar to the slow cooker and stir to combine. Cover with the lid and cook on low for 7-8 hours overnight.
In the morning, remove the lid and stir the oats. Sprinkle the brown sugar over the top and let sit.
In a small bowl, combine the cream cheese frosting ingredients. Using a hand mixer, beat the ingredients on low until you have a creamy frosting. Add additional milk if the icing seems too thick.
When ready to serve, hand pipe or drizzle the icing over each individual bowl of the slow cooker steel cut oats and serve immediately.