When we think of mustard, we usually think of the pungent, vinegary brown or yellow condiment. There's much more to mustard seeds, however, than prepared mustard.
When you're cooking with mustard seeds, you must first determine which mustard seeds you wish to use. This can get confusing, because the different varieties of mustard seed are inconsistently named. The most common variety of mustard seed in Europe and America is yellow mustard, also known as white mustard. Indian cooks generally use black mustard seeds, which are smaller and have a more intense flavor; these are sometimes sold as brown mustard seeds. A third variety is commonly known as hot mustard or Oriental mustard, but is also sometimes called brown mustard or even hot yellow mustard.
Ground, raw mustard seeds have a sharp, spicy flavor when mixed with liquid. You can make your own mustard by grinding whole mustard seeds into a powder, then mixing it with water or vinegar.
When you use whole mustard seeds in cooking, however, you get a very different flavor profile. Indian cooks usually fry mustard seeds when starting out a dish; this brings out sweeter notes in the mustard seeds' flavor.
To cook with whole mustard seeds, coat the bottom of a heavy pan with a little vegetable oil or ghee (clarified butter). Let the oil get very hot, and then add the mustard seeds. They will quickly start to sputter and pop. Stir the mustard seeds constantly and fry them for about 10 seconds, then remove from the heat immediately. Overcooked mustard seeds develop a bitter, oily taste.
You can now use the mustard seeds in stir-fries and other recipes. For an easy side dish, start with 1 teaspoon fried mustard seeds and add 3 cloves minced garlic to the pan. Cook for 30 seconds, and then add 5 ounces baby spinach and cook until it wilts. Finish with 1-tablespoon rice wine, 1/4-teaspoon rice vinegar and a pinch of salt.