A fiddlehead is the tightly coiled young frond, or crozier, of the ostrich fern (Matteuccia struthiopteris), foraged for only a few weeks each spring before it unfurls into a leaf. Bright green and spiral-wound like the scroll at the head of a violin, each tender curl carries a thin papery brown husk and tastes grassy and vegetal — somewhere between asparagus, green bean and spinach. It is a celebrated springtime delicacy of New England and eastern Canada, but one that must always be thoroughly cooked before it is eaten.