To preheat the Instant Pot, select the Sauté function. When display reads "hot", add the butter and melt. Add the oats and stir, cooking for 2 minutes. Add the water, apple, brown sugar, cinnamon, and salt and stir. Secure the lid. Select the Manual function and cook at high pressure for 7 minutes. Once cooking is complete, use a natural release for 10 minutes, then release any remaining pressure quickly. The oatmeal will continue to thicken as it cools. Serve the oatmeal topped with chopped fresh apple and more brown sugar. Enjoy!