Bring the water and milk to a simmer in a large saucepan over medium heat.
Meanwhile, melt the butter in a skillet over medium heat, then add the oats and toast, stirring constantly, until golden and fragrant, 1 1/2 to 2 minutes.
Stir the oats into the simmering liquid, reduce the heat,to medium-low, and simmer gently until the mixture is very thick, about 20 minutes.
Stir in the salt and the optional add-ins and continue to simmer, stirring occasionally, until almost all of the liquid has been absorbed and the oatmeal is creamy, about 10 minutes. If doubling the recipe, increase this to 15 minutes. Let stand for 5 minutes before serving.