Pour 3/4 cup of water into the Instant Pot.
Chop the peeled apples into 1-inch cubes, and place them in a large bowl. Sprinkle the apples with the flour, cinnamon, and salt. Stir to coat evenly, then stir in maple syrup.
Transfer the apples to the inner pot of the Instant Pot that has the 3/4 cup water in it. Using the same bowl, prepare the topping by stirring together the oats, cornmeal, walnuts, cinnamon, and salt. Add the maple syrup and coconut oil. Stir together.
Spoon the topping evenly over the apples. Cover the Instant Pot, turn the knob to Sealing, and set the pot to cook for 1 minute at manual high pressure. Once the cook time is done, do a quick release. It will be very hot at this point, so let it cool a bit before serving. This can be served warm, or you can refrigerate it until ready to serve. This apple crisp will keep for several days in an airtight container in the refrigerator. It can be eaten cold, or you can reheat it in the oven.