Mustard greens are a very hardy plant which can be grown in the winter under a polytunnel or greenhouse. They do not need a lot of light to grow. I have grown mustard greens during the summer, and I found the taste to be much stronger and peppery during the warmer seasons. In the winter they tend to have a more mild taste.
When you are harvesting the leaves be sure to let a reasonable amount of the plant and its leaves remain, as this will allow more leaves to regrow for harvesting in future.
Here in my polytunnel I have managed to grow and harvest quite a decent crop, which I am very happy with. I found that these plants are quite hardy and are a good choice for people who may be looking to start their garden during the colder time of the year, and is looking for a quick turn around.