In a small bowl that fits inside you Instant Pot, mix together the oats, milk, 2 cups of water, and cinnamon stick. Pour the remaining 1 1/2 cups of water into the inner pot, and place the trivet inside. Place the bowl on the trivet, and lock the into place.
Select Pressure Cook or Manual, and adjust the pressure to High and the time to 5 minutes. Make sure the Steam Release knob is in the Sealed position.
After cooking, naturally release the pressure for 10 minutes, then quick release any remaining pressure.
Unlock and remove the lid. Remove the bowl, and mix the oats thoroughly. Stir in the sugar, 1 chopped apple, ground cinnamon, and walnuts. Top with the remaining 1 chopped apple and serve warm.
Notes
Make it Vegan: replace the milk with almond milk, soy milk or your favorite plant-based milk