Shimeji mushrooms
Small beech mushrooms with firm stems and round caps that grow in clusters, brown or white, with a mild, nutty flavour and a slight crunch.
Shimeji are now sold in many larger supermarkets, often labelled beech mushrooms, and otherwise in Asian grocers; trim off the shared base and pull the clusters apart.
Substitute:
Oyster mushrooms torn into strips come closest in firm bite and mild flavour; button mushrooms also work but turn softer and taste less nutty.