This small cluster of mushrooms appeared recently in a garden bed amoungst some horse manure.
These mushrooms have a white cap with a brown colouration in the middle and when we flip them over, we can see that the gill colour is in the right range.
However, if we try to separate the cap from the stem, we find that we can’t, and the stem is furthermore completely hollow and thinner than what we might expect from an Agaricus. The mushroom pictured is Psathyrella candolleana. There is another species more commonly seen in forests called P. aspersopora. Both are of unknown or doubtfull edibility.