A "Gastroid Pholiota".
This is probably a species of Nivatogastrium. David Arora's book Mushrooms Demystified describes a species called N. nubigenum, but that is said to occur near mountain snowbanks on rotting conifer, whereas this species appears to be a desert-loving terrestrial. Arora mentions another species, N. wrightii, but I can find no information about it to see how it compares with this specimen.