Leaf mustard is grown, as its name suggest, for green salad leaves (rather than the seed).
It is extremely easy to grow and will germinate within a few days if sown straight into the ground in spring.
You can harvest the young tender leaves for salads within around 30-40 days.
Just pick off the outer leaves and one plant will continue to sprout new ones and give you continual cropping.
The leaves when older can also be cooked like spinach. Either way they have a fresh but peppery taste. If left they will often 'bolt' to seed - in which case they taste awful.
They are more prone to bolting to seed in warmer climates in Australia.