Stir the oats into the simmering water/milk mixture. Reduce the heat to medium low and simmer gently for about 20 minutes, stirring occasionally, until the mixture is very thick.
In a large saucepan, combine the water and milk. Bring the mixture to a simmer over medium heat. In the meantime, melt the coconut oil (or butter) in a 12-inch skillet over medium heat. Once shimmering, add the oats and cook, stirring occasionally, until golden and fragrant, around 2 minutes. This toasting step greatly enhances the flavor of the oats.