Bring water to a boil in a medium pot. Add the oats and the salt. Allow to simmer until about 80% of the water has been absorbed, about 20 minutes, stirring occasionally.
Stir in the milk, mashed bananas, maple syrup and cinnamon, whisking to combine if needed. Return to a simmer and cook, stirring occasionally, until the oatmeal reaches your desired consistency, about another 10-15 minutes.
Remove from heat and stir in the walnuts and vanilla.
Serve with additional banana slices, cinnamon, chopped walnuts, maple syrup, or honey as desired.
Notes
Use any kind of milk you like - almond, soy, or coconut milk for a healthier, non-dairy option, or whole milk for a richer oatmeal.Makes about 5 cups of cooked oatmeal.