© Denzil Green
A fiddlehead is the top of a fern when it first pokes through the ground in spring, curled up like the head of a fiddle. They are only available for 3 weeks in May. They are still mostly gathered from the wild, though commercial production is being explored.
They taste something like asparagus and spinach.
The most popular fern to harvest them from, and the one that is deemed the safest, is the Ostrich Fern. (see Nutrition below.) Ostrich Fern fiddleheads will have on them brown, damp "chaff" that feels papery or "fuzz" that feels like down.
Fiddleheads are very popular throughout Asia and the Pacific, particularly in Japan and in Korea, and popular in eastern parts of North America. In Asia they are cooked; in Hawaii, just parboiled.
When buying fresh fiddleheads, choose luminous, bright green ones that are small -- no more than 5 cm across and 5 cm tall (2 inches x 2 inches) including the stalk.
Fiddleheads can also be bought canned.
Fresh, they only have a shelf life of about 3 days.
Trim stalks to 1/4 inch (1/2 cm). Rub any scales or brown fuzz off, and rinse.
Bring salted water to a boil, add fiddleheads, boil for 15 minutes, or steam for 10 to 12 minutes. 
These are the only two acknowledged safe ways and times by food safety experts to cook fiddleheads, to leech the toxins out. Besides, they really do need the full cooking times to bring the flavour out.
Drain; discard the cooking water.
Afterwards, you can do further cooking such as sautéing, etc, or simply serve garnished with lemon, butter or a vinaigrette.
To be clear, if you are planning to sauté or otherwise cook them, you must still steam or boil them first to leach any toxins present in them out. The boiling or steaming for the prescribed times are required food safety steps.
Some experts advise not to eat any fiddleheads at all.
Health Canada advises to discard any steaming or simmering water that you cooked fiddleheads in, as toxins may have leeched into the water. Health Canada and the American CDC both advise the cooking time of 15 minutes for boiling, and 10 to 12 minutes for steaming, to leech out the toxins.
There are no safe, tested recommendations for home canning plain fiddleheads, but the University of Maine Extension Service has developed some recipes for pickling fiddleheads.
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FernsFiddleheads; Warabi; Zenmai
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-- Clifton Fadiman (American author & editor. 15 May 1904 – 20 June 1999)