Start by mashing the ripe bananas in a mixing bowl until smooth. Measure out the steel cut oats, water, milk, ground flaxseed meal, vanilla extract, ground cinnamon, freshly grated nutmeg, and kosher salt.
Add the steel cut oats, water, milk, mashed bananas, flaxseed meal, vanilla, cinnamon, nutmeg, and salt into your slow cooker. Stir everything together until well combined.
Cover and set your slow cooker to low. Cook for about 7 to 8 hours overnight. The slow, gentle heat will transform the steel cut oats into a creamy, porridge-like consistency.
In the morning, give the oats a good stir. If the texture is too thick for your liking, simply stir in a splash of milk or water. Serve warm topped with your favorite mix-ins like toasted walnuts, raisins, chocolate chips, or a drizzle of maple syrup.