In the inner pot of the Instant Pot, place the steel cut oats, milk, water, shredded apples, cinnamon, and salt. Lock the lid in place and switch the valve to seal.
Set the Instant Pot to cook on high pressure, manually adjusting the time to 6 minutes. It will take about 10 minutes for the Instant Pot to build up the pressure before the timer starts counting down.
When the timer goes off, allow the pressure to naturally release for 10 to 20 minutes. Do not switch the valve to ‘vent’ until after the natural release time is up.
Once the steam is fully released, remove the lid and stir in the vanilla extract and whey protein powder.
Serve the steel cut oats with toppings of your choice such as chopped apples, toasted pecans, a drizzle of maple syrup, or additional cinnamon.