Turn on the Saute function of your Instant Pot and melt the butter.
Saute the onion slices in the melted butter with the salt, cinnamon, and nutmeg, stirring every so often, until the onion is soft - about 5 minutes.
While the onion is in the Instant Pot, peel, seed, and slice your squash. (A melon baller works very well to get the seeds out.)
Add the chicken stock/water combination and the squash slices. Stir to combine.
Cancel the Saute function, put the lid on your Instant Pot and turn it to the lock position. Make sure the valve is set to Sealing and push the Manual button. Adjust the time to 10 minutes (using the - and + buttons) and let it cook.
When the ten minutes is up and the Instant Pot beeps, manually release the pressure by canceling the warm setting and turning the valve (be careful as HOT steam will come out at this point) to Venting. Wait until it is completely vented and the red Float Valve has gone back down, indicating that it is no longer under pressure.
Remove the lid. You can also take the pot out and set it on a trivet, though this is not necessary. You can also let it cool for a bit before you blend it, but I don't normally do this.
Use your immersion blender to blend the soup to the desired consistency (my kids like it a little lumpy). You can serve it immediately or wait a bit to cool it off. I usually stick an ice cube in the bowls of soup I serve to my kids so it cools off faster.