Butter a round 2-quart casserole dish.
Whisk together all ingredients except the apples. Peel, core and dice the apples and stir into oatmeal.
Pour oatmeal into prepared dish. Cover with plastic wrap and then aluminum foil. Crimp the foil around the dish securing in the plastic wrap and creating a good seal.
Set a support trivet at the bottom of the cooking pot and add one cup of water.
Lower the prepared dish onto the trivet.
Secure and lock the lid. Set the pressure valve to HIGH pressure.
Begin heating on high to build pressure. Once steam starts to come out of the pressure valve and handle, reduce heat to maintain a steady stream of steam. Start cooking time of 30 minutes.
Once cooking time is complete, remove the pot from heat. Allow the pressure to naturally come down, do not release the pressure manually.
Carefully remove the dish and allow oatmeal to rest for 10 minutes.
Scoop or cut into wedges and serve with maple syrup, yogurt or cream.