Nettle soup has long been eaten as a spring tonic. Look for nettles at farmers' markets throughout the spring and early summer. Nettles are covered with nearly invisible sharp hairs that can sting painfully, but cooking renders them harmless. Giana Ferguson, the artisanal cheese maker who gave us this recipe, advised us to wear rubber gloves when handling nettles. Wash them well, and use only the tips and tender leaves, snipping them off with kitchen shears.