Combine the rolled oats, liquid, and salt in a medium saucepan over medium-high heat.
Bring the mixture to a boil, then immediately reduce the heat to low to maintain a simmer.
Cook for 5 minutes, stirring occasionally to prevent bottom scorching and to release oat starches.
Incorporate the mashed banana and ground cinnamon into the saucepan, stirring vigorously to distribute.
Continue to simmer for 2 to 3 minutes until the oatmeal reaches a thick, creamy consistency.
Remove from heat and stir in the maple syrup and vanilla extract until fully emulsified.
Portion into bowls and garnish with chopped walnuts before serving.