Nameko fungus. なめこ。Pholiota nameko. Sometimes called "butterscotch."
This mushroom is unique to Japan. It is golden-brown in color and has a slippery, gelatinous coating, a pleasant contrast. The sliminess may surprise some customers abroad, but nameko really are delicious! The taste is mildly nutty.
Nameko is used in miso soup, in hot-pot dishes (nabemono) and in aemono (cooked salads). They can also be added to the soup of soba or udon - popular is "nameko-soba," where the soba noodles are eaten cold but covered with a mixture of nameko and daikon-oroshi (plus some shredded spring onions or shiso).
Does not keep well, so usually sold canned or bottled; in Japan also fresh, in plastic pouches. Rinse before use.