With a large sharp knife, carefully slice off the top of the pumpkin, about 1 inch under its stem, then cut the pumpkin in half vertically. Remove the seeds and stringy innards from the pumpkin's center, using a large spoon and a small paring knife.
Pour 1 1/2 cups water into Instant Pot, and place the cooking rack. Arrange the pumpkin halves to fit, taking care not to block the pressure valve area. If the pieces are too large, slice the halves into quarters.
Close the lid and ensure that the valve is set to sealed position. Select "Steam" mode, leave the setting on High Pressure, and adjust the time to 15 minutes.
When the cooking time is up, the pressure valve may be opened immediately for a quick release. The pumpkin flesh should be very soft and easy to spoon out of its skin (if it isn't, close the lid and cook again for a few minutes more as needed). Scoop the pumpkin flesh into a large bowl, discarding the skins.
To puree, you may use a food processor or blender - or to produce fewer dirty dishes, return the pumpkin to the Instant Pot (after removing the rack and pouring off the cooking liquid) and use an immersion blender to puree until smooth.
To drain excess liquid from the pumpkin puree, place a mesh strainer over a bowl, and line the strainer with a coffee filter or cheesecloth or paper/cotton towels. Mound the pumpkin puree into the lined strainer and allow it to drain into the bowl for at least an hour, or in the refrigerator overnight.
Store the pumpkin puree in an airtight container; use within four days if refrigerated, or freeze for long-term storage.