The most hardy winter greens in my garden are mostly rich in sulfur compounds including various onions (Allium) and onion-tasting brassica, Garlic mustard (løkurt), Alliaria petiolata or is it just coincidence that Alliums and Alliaria are both very hardy.
Does the sulfur play a role in giving these plants hardiness? The only other plant that I can harvest a little of in midwinter is Hablitzia tamnoides (although it turns red in the coldest weather).