We had a fun surprise waiting for us in our CSA box this week.
I thought the thin stalks looked fern-like, but it didn't quite resemble the fiddlehead ferns I'd had tempura-style a couple of times here. So I e-mailed the very kind Kat at Our Adventures in Japan for assistance. She confirmed that they were indeed fiddleheads, known as warabi in Japanese. Thanks Kat!
They were wrapped up with a little package of baking soda, and after a little googling I learned why. According to this blogger, "a bit of baking soda is used when blanching this type of spring mountain greens, which slightly softens them and also removes more of the traces of enzymes that, given long term heavy consumption of the plant, can lead to some health problems."