Prepare the lemon water. Fill a large bowl halfway with cool water and squeeze in the juice from half a lemon.
Prepare the apples. Peel, core and cut the apples into approximately 1/4-inch-thick slices. You should have approximately 6 cups. Place the apple slices in the lemon water as you work to prevent browning.
Fill the Instant Pot. Drain the apples well and add them to the pot. Add the apple cider, granulated sugar, brown sugar, butter, cinnamon, nutmeg and salt. Stir gently to coat the apples and press them down into the pot.
Pressure cook. Secure the lid and set the steam release valve to the sealing position. Cook on high pressure for 3 minutes. The Instant Pot will take approximately 17 minutes to build pressure before the cooking time begins.
Release the pressure. When the cooking time is complete, carefully perform a quick release, keeping your face and hands away from the steam. Once the pressure has fully released, remove the lid.
Thicken, if desired. For a thicker, more traditional apple pie filling, select the Sauté setting. Whisk together the cornstarch and cool water until completely smooth. Gently stir the slurry into the apples and simmer for 1 to 2 minutes, until the sauce thickens. Press Cancel. The filling will continue to thicken as it cools.
Cool and use. Allow the filling to cool before adding it to a pie or another dessert. One batch makes enough filling for one apple pie, or 8 servings. Use it immediately, refrigerate it in an airtight container for up to 5 days or freeze it for up to 6 months.