Mustard is hot. It's not only spicy, but also hot stuff at the cash register. Mustard is right up there this year with boutique California wines, vegetable pasta, gourmet pizza and vintage olive oil.

On the other hand, there is a sharp difference between mustard and more recent food fads: we Americans know mustard; according to Simmons Market Research Data, 90 percent of us have used it since childhood. Americans spend about $150 million each year on mustard and the numbers have been multiplying geometrically. About 33 percent of us go through at least a jar of mustard a month. And not just the ballpark stuff. The 1981 Simmons survey discovered that about 15 percent of today's mustard-users in the U.S. favor dijon-type mustard.

Nevertheless, we have a lot to learn about mustard from the Europeans, who have a mustard for every region and every occasion. It turns out that the average American consumes about half a pound of mustard each year, on contrast with the average Frenchman, who east almost two pounds every year, reveal Sally and Martin Stone, the authors of "The Mustard Cookbook."

Unlike the cheeses and salamis they may grace that are made specifically for the export market, mustard is one of the few fancy imports that can be enjoyed here in exactly the same form as it is in Europe, assuming it arrives relatively fresh. When the Burgundy district in Franch or Dusseldorf in Germany send us their mustards, they are sending us exactly the same jars that might be brought by shoppers in their own neighborhoods to serve with pot au feu or weisswurst.

According to Vincent P. Dole, president of Dolefam Corporation, importers of Old Monk mustard, mustard is made from white and/or black seeds, which come from different species of the mustard plant, brassica. The white (also called yellow) are milder than the black (also called brown) seeds, with a sharp aftertaste. Almost all American mustard are made from white seeds, which is why they are not as pungent and complex as dijon and other imported mustards.

More mustard is sold around the world than any other spice, including pepper, the R.T. French Co. brags in its literature. The U.S. spends about $5 million per year on imported prepared mustard. One third of the imported mustard is from France, mostly from the city of Dijon, says Dole. The rest of the world buys two-thirds of its mustard supply from Dijon.

There's good reason for the monopoly: in France in 1634, King Louis XIV granted the 23 mustard-makers of Dijon the exclusive right to make dijon mustard for sale. Even today, French law carefully governs the manufacture of dijon. The mustard is controlled by French law in much the same way as French wine, reveals Food and Wines from France, a trade organization which represents mustard. It is made from only mustard seeds, white wine, vinegar and spices. Manufacturers must not add fillers such as flour or sugar; amplifiers such as mustard oil; chemical preservatives; or coloring.

Dijon mustard is made by grinding black and brown mustard seeks (no white is permitted) and mixing them with salt, water and vinegar. The hulls are strained away and spices and wine (or wine vinegar) are added. Many dijon mustards are aged in wooden casks.

Literature from Foods and Wines from France enumerates French law on the subject of mustard: if a Dijon mustard contains white seeds, it must be called "condiment" rather than "mustard." If the word forte is on the label, it contains at least 28 percent mustard powder; extra-forte is, of course, extra strong.

The only mustard that can be called dijon by French law is from Dijon, with one exception: a few years ago Heublein bought the rights to make Grey Poupon Dijon mustard in California, and is allowed to carry the name, even though the product is much milder than its Dijonnais forebears. The French Grey Poupon is no longer available here.

Being French and very serious, the formulas for the mustard which is made under more than 80 labels in Dijon are always kept secret. And looking at the label of a French mustard will not give you a clue about its taste: the same ingredients appear again and again; the only way to decipher differences among the brands is through experience.

Some of the other mustards available in American markets are German, English and coarse-grained. Some people feel that only dijon works well as a cooking ingredient, but each is an appropriate condiment for cold meats, cheeses, sausages and the like.

German mustards are generally made from strong mustard powder and vinegar. There are two styles of German mustards: Bavarian, which is sweet and dark, and Dusseldorf, which is more spicy.

English mustard is made of white seeds and is characteristically pungent and straightforward.

Coarse-grained mustard, sometimes called old fashioned or a l'ancienne or farmhouse mustard, is made the way mustard was produced before the 18th century -- with coarsely ground seeds left in the mixture. It is generally sweet and musty.

If you are daunted by the prospect of sampling the countless mustards available today, we have done the work for you. After gathering and tasting almost 150 mustards (using celery sticks, matzos, plastic spoons and fingers), we offer a list of those we found worthwhile. You will not see ballpark mustards on the list, on the assumption that you know from long experience which of those you prefer.

One general word of advice: we found the flavored mustards to be of poor quality, and expensive to boot. If there is a flavor you fancy, make it yourself by adding 2 tablespoons or so of an ingredient to about 1 cup of good-quality dijon. Good flavorings to try are tomato paste, parsley, honey, lemon and horseradish.

Here is a rundown of recommended mustards:

In the dijon and dijon-styles, we found Recorbet to be very sharp and biting; the Old Monk is a good, all-around dijon. La Charcuterie, the least expensive of the dijons, tasters found sharp, hot and vibrant.

In the coarse-grained category, Recorbet Old Fashioned was judged on the strong side; Maitre Jacques Grained pungent and complex. Tasters also liked Elsenham Whole Grain, an English mustard and the most expensive of the three.

Recommended German mustards were Hengstenberg Extra Hot (which turns out to be tasty, though not extra hot), FS Bavarian Sweet and Lion Dussendorf.

Flavored mustards that tasters liked included Maitre Jacques Green Peppercorn, Maitre Jacques Lemon (powerful stuff), Maille Green Herbs, Crabtree and Evelyn Green Pepper (the most expensive of the group) and Maitre Jacques Shallot, an especially fiery mustard.