Pour the water and milk into a saucepan and bring to a gentle simmer over medium heat, releasing a warm, inviting aroma.
Add the rolled oats to the simmering liquid, stirring constantly to prevent clumping, as the mixture begins to thicken and bubbles form around the edges.
Stir in the diced apple, ground cinnamon, ground cardamom, brown sugar, and a pinch of salt, allowing the spices to distribute evenly and the apples to soften in the hot mixture.
Reduce the heat to low and let the oatmeal simmer gently, stirring occasionally, until the oats are tender and the mixture has thickened to a creamy consistency, about 8–10 minutes.
Taste and adjust sweetness or spices as desired, adding more sugar or a pinch of cinnamon for extra warmth.
Remove the saucepan from heat and let it rest for a minute to settle and develop extra creaminess.
Spoon the steaming, fragrant oatmeal into bowls, garnishing with additional apple slices or a sprinkle of cinnamon if desired.
Enjoy the warm, silky oatmeal right away, savoring the cozy flavors and tender fruit pieces.