In a small saucepan, combine the water, milk, diced apple, cinnamon and salt. Bring to a simmer over medium heat.
Stir in the oats and reduce the heat to medium-low.
Cook, stirring every minute or so, for 5–7 minutes, until the oats are tender and creamy and the apples have softened.
Remove from the heat and stir in the maple syrup, vanilla and butter, if using. Let stand 1 minute; it will thicken a little more.
Divide between two bowls, add a splash of milk if you like it looser, and top with the toasted nuts and an extra pinch of cinnamon.