In the Indian subcontinent they are often used whole, and are quickly fried in oil until they pop to impart a flavour to the oil.
Mustard oil can be extracted from the seeds. Mustard oil has a strong smell, a little like strong cabbage, and is much used for cooking in Bengal.
The seeds, particularly the white ones, can be ground into a flour, and mixed to a thick paste with a little water to make the condiment mustard. The ground mustard powder is usually mixed with ordinary flour to reduce the strength of the resulting condiment.
Other ingredients can be used to mix mustard, for example, sugar, honey, vinegar, wine, or milk.
When initially mixed the sauce is mild in flavour, but it develops in time. Strong mustard has a very powerful (and painful) effect on the nasal membranes if eaten carelessly.
The whole seeds can be soaked in liquid before grinding to create whole grain mustard.
It is possible to buy ready prepared mustard in many places, but the freshly prepared product is usually far superior.
According to the Canadian Food Inspection Agency, mustard oil is "not suitable for human consumption".[1] Mustard oil is composed mostly of the fatty acids oleic acid, linoleic acid and erucic acid.[1]