Kale (Brassica oleracea var. sabellica) is a hardy, non-heading leafy brassica grown for its loose rosette of large, ruffled or curly blue-green leaves rather than any tight head — the most ancestral, cabbage-like of the cole crops. Grassy, peppery and frankly bitter when raw, with the sulfurous, cabbagey edge shared by all its kin, it turns sweet, nutty and tender after a hard frost or a few minutes of cooking. Long a humble winter staple of northern Europe — the everyday green of Scotland, the Netherlands and northern Germany — it was reinvented in the 2010s as a global "superfood," eaten massaged into raw salads, crisped into chips, blitzed into smoothies and stewed into soups.