Fiddlehead ferns are a favorite spring ingredient. They are the furled fronds of a young fern. They have a short season, and their flavor has been compared to other green vegetables including artichokes, asparagus and green beans. When choosing them, look for fiddleheads that are small, firm and tightly furled. You should store them in the refrigerator wrapped tightly in plastic. They won’t spoil quickly, but they will lose flavor and firmness after a couple of days. The extra bite from the walnuts and umami flavor from the anchovy fillet makes this a lovely sauce for grilled vegetables, pasta and roasted potatoes.