In a medium saucepan, add water, milk, oats, salt, sugar, cinnamon, and cream of tartar.
Heat over medium-high heat until oats begin to bubble. Turn heat to just below medium and continue to simmer, stirring occasionally until oats are desired texture and thickness (they will further thicken upon standing) around 4-5 minutes.
Portion into bowls and serve warm with an extra sprinkle of cinnamon and sugar if you want. Enjoy!