Place the trivet in the bottom of the Instant Pot and pour in ½ cup water.
Arrange the peeled, ½-inch-thick yam slices on top of the trivet.
In a small bowl, stir together the brown sugar, salt, cinnamon, and nutmeg.
Sprinkle the brown sugar mixture evenly over the yams, then top with the sliced butter.
Secure the Instant Pot lid and set the valve to the Sealing position. Cook on High Pressure for 8 minutes.
When the cooking time is complete, carefully perform a quick release.
Remove the lid and carefully remove the trivet. Gently toss the yams with the butter and brown sugar mixture until evenly coated.
If the sauce is too thin, stir together 1 tablespoon cornstarch and 1 tablespoon water. Stir the slurry into the sauce and use the Sauté setting for 1–2 minutes, stirring gently, until thickened.