Mustard greens. Common in Greece, where the leaves are eaten in the winter months and early spring before the plants flower. The are so common in fact that the expression, 'Πήγε για βρούβες' ('Going to fetch mustard greens') means a pointless journey because they're everywhere. Cretans make them into pies, βρουβόπιτες (vrouvopites).
Another name for it is λαψάνα (lapsana).
Image by Stacy Spensley