Dump the rice, oatmeal, milk, cardamom, salt, water, and sugar in a large saucepan set over medium heat and stir to combine.
Let the ingredients warm until the liquid starts to bubble around the edges of the pan. Reduce the heat as necessary to maintain a very gentle simmer and cook, stirring and scraping the bottom of the pan frequently so the grains don’t stick, until the rice is broken down and the porridge has a soupy consistency, 60 minutes. You want to be careful to not let the porridge boil.