In a large pot, bring water and milk to a boil (watch closely to make sure that it does not boil over). Once boiling, add the steel cut oats, salt and cinnamon and stir to combine.
Bring to a simmer over low heat and cook, stirring occasionally, for 20 to 25 minutes until the oats are creamy and tender. Serve immediately with desired toppings. Stores refrigerated up to 1 week: it will become very thick when chilled, so stir in some milk or water when reheating.