Turns out both grow in my area, and look identical to this when young. Yikes! So based on a post yesterday, I took this outside and sliced it in half. So far it looks promising (I think?) and I’m not dead yet.

This was found growing in a Colorado yard near the base of an elm tree, in an area where there are also rotting cottonwood roots. Altitude is right at 5000 feet. It wasn’t my yard so I’m not sure how many days it may have been growing before I picked it today. I have put both halves in the fridge for now, is there any other information I can provide to help identify it?

A full size copy of the inside can be viewed here: http://sourpuss.net/projects/mycology/2023-08-13/IMG_7239.JPG