Our maid planted this plant for her dishes (in VAS office).  
In Thailand they use this vegetable for soups.

Here is description written by Ms Susan Strand-Nelson.

The mustard family is large, with 375 genera and 2,000 species worldwide.  Brassica is found circumpolar.  It was introduced to North America from Asia.  The family name, Cruciferae, indicates the petals of the flower number four, like a cross or crucifix.

Missionaries, in the Americas, planted the cross-shaped flowers to show the path from one mission to the next.  The name ‘mustard’ comes from the Roman practice of soaking the seeds in newly fermented grape juice, called ‘must.’  This was drunk by the armies in preparation for battle.  Mustard oil obtained from the seeds of Brassica rapa was used by the Chinese for lighting oil before the introduction of kerosene.

There are many food and medicinal uses of the wild mustards.  The leaves can be added to salads, egg dishes, and sandwich  fillings.  They can be served as a potherb.  North American natives enjoy greens that have been fermented like sauerkraut.

Medicinally, mustards are a stimulant for the gastrointestinal tract, an antiseptic for wound cleansing, and a rubefacient for drawing blood to the surface.

The seeds taken internally in large amounts have been known to poison cattle.  If the pure oil is applied to the skin it can cause severe blistering.  

For identification: Flowers have 4 yellow petals. Stems grow 1-4 feet high. Pods are narrow, upright (Siliques) with many seeds. Lower leaves variable with species, smooth or pubescent, deeply lobed or with entire margins. Stem leaves clasping for B. rapa and B. napus. Stem leaves not clasping for B. juncea.

References
The Flora of South Central Alaska by Boyd Shaffer
Discovering Wild Plants, Alaska, Western Canada, the Northwest by Janice Schofield
Flora of Alaska and Neighboring Territories by Eric Hulten

 - Susan Strand-Nelson