Finely ground mustard powder has such a harsh bitter flavour that, unlike other seeds -- sesame and caraway come to mind -- it is never eaten by itself. However, mustard seed as a food additive is one of our oldest continually used spices. Traditionally, mustard is paired with meat, perhaps because its spiciness and astringency increase the flow of saliva which helps digestion. Its flavour is most pronounced in the familiar yellow mustard, particularly when prepared English style in which water and mustard seed powder are mixed into a thick paste.
Black mustard seeds can also be ground into a powder but the flavour is bitterer, less mustardy. Most people who taste it for the first time dont like it. These seeds are often called for in pickling recipes; but their most common use is in the grainy French style mustards that go so well with patés. And that might be the only way that we would eat black mustard seeds if their flavour didn't change so impressively when roasted or fried.
Try them this way and cooking becomes a complete sensory experience. First heat some vegetable oil until it starts to smoke. Add a spoonful of black mustard seed and you'll soon hear them pop. Stir them quickly and remove them from the oil as soon as the seeds turn gray. Let them cool a little before tasting. Now the flavour of the seeds has changed from bitter to nutty. Their mustardy heat is only barely noticeable. They look and taste a lot like slightly smoky poppy seeds.
Here is a simple Indian-style dish that shows off this flavour. Cook a cup of rice. When it is ready, put a frying pan or wok over medium-high heat and wait until the pan is hot. Put in a quarter of a stick of butter. When this has melted and sizzles add a teaspoon each of black mustard seed and turmeric and a half teaspoon of salt. Stir this mixture until the seeds pop and turn gray; add the rice. Mix this well and squeeze in the juice from half a lemon. Stir again and serve.