In a medium-small pot over medium heat, bring the milk to a simmer.
Stir in the oats, mashed banana, and cinnamon.
Simmer for 30 minutes, or until the liquid has mostly been absorbed and the oats are tender. Stir occasionally.
Remove from heat, then stir in the salt, vanilla, maple syrup, almond butter, and walnuts. Taste, then adjust the ingredient amounts to your liking.
Serve with banana slices, more walnuts, and berries, if you wish.