To begin, to a medium-sized pot add your water and milk and bring them to a boil on high heat.
Add your steel cut oats and mashed banana, and reduce the stove to medium-low heat. Cook for 20-25 minutes, stirring occasionally. The oats should be tender enough that they aren't hard to chew, but they will be a little chewy.
Take your pot off the heat, and add your cinnamon and vanilla. Stir till everything is well combined.
Serve plain or with chopped walnuts, chocolate chips, or a scoop of peanut butter. Devour!
Stored in an airtight container in the fridge, these oats should last about 5-6 days.