A Chinese Cabbage
You'd have seen one of these cabbages before in markets but it is never this big. This is a type of Chinese cabbage, the Napa cabbage or Brassica rapa subsp. pekinensis - pekinensis - most probably because it originates near the Beijing region. The bok choy is a Chinese cabage too but it is darker in colour. The Napa cabbage is widely used in China, Japan and Korea and when the inhabitants of these countries migrated to other countries, they brought along the Napa cabbage with them. Now, it is readily found in many North American, European and Australian cities. Good things should be spread around!