CONSIDER the scallion.

Geneticists at the Agricultural Research Center in Beltsville have been doing just that every day for the last four years. They've been breeding out the floppy leaves and big bulbs of the locally developed Beltsville Bunching Onion (BBO), and breeding in resistance to diseases and insects.

Before 1950 and the discoveries of Henry Jones, the "father of hybrid onions," scallions raised on the East Coast were prematurely harvested standard white bulb onions, says J. D. McCollum, onion research geneticist at the center. Since then, however, scallion farmers in southern New Jersey, the largest scallion farming area close to Washington, have switched to Jones' BBO, a hybrid of the familar white bulb onion and a Japanese bunching onion.

"What's interesting about the BBO ," says McCollum, "is that it was the first hybrid scallion ever bred from the white bulb onion and a non-bulbing bunching parent." Jones' new breed, for a time, contributed resistance and produced stronger, more prolific, better-looking offspring. The Japanese parent provided slenderness and resistance, while the bulb onion made it more vigorous.

As it turned out, the BBO's success was temporary. Small-scale farmers who raised well over 10 million bunches of these scallions in 1981 and distributed them up and down the East Coast are once again voicing concerns, according to McCullum. They want the researchers to build a better onion.

"It's been 30 years since the BBO was first released," McCollum says. "Whether they don't remember it or never knew it, or maybe even some natural selection that has changed the characteristics, we'll never know. We don't have any seeds from the first release back in 1950."

So when the New Jersey Extension Service called the Beltsville research center about the problem, McCollum went to work on the case. Now, a portion of each workday is devoted to studying the genetics of the BBO and some 10 to 15 new strands of resistant scallions. Because he's still on the first generation of new scallions, which are infertile, it'll be several years before we see them in the seed catalogues.

One thing he's not breeding out, though, is the sweet, sharp taste of this vegetable used across the continents in almost every cuisine imaginable. Raw or cooked, it is one of the most important vegetable flavorings growing.

To buy them, look for the smallest, firmest bulbs with firm, deep green tops. They hold well in the vegetable crisper up to three days--any longer and they begin to soften and lose pungency.

Serve them raw, tied in knots to dip in peanut sauce, or as a colorful topping to a curried cheddar cheese ball frosted with mango chutney. For a main course, stir-fry a little lamb with a lot of chopped scallions and serve it with steamed white rice. ONION KNOTS WITH PEANUT SAUCE (Makes 2 to 3 dozen appetizers) 2 to 3 dozen scallions (including tops) 12 cups water For the peanut sauce: 1/4 cup chunky peanut butter 1/4 cup plum jam 1 tablespoon lemon juice 1 tablespoon soy sauce Hot pepper sauce to taste

Rinse scallions and cut off roots. In a 5- to 6-quart kettle over high heat, bring water to a boil. Add scallions a few at a time, and cook just until green ends appear limp (about 20 seconds). Lift out and plunge immediately into cold water; when cool, place onions on a cloth and let drain. Pull off and discard tough outside layers of each scallion. Tie each in a loose knot. At this point you may refrigerate for up to 6 hours.

Combine peanut sauce ingredients in a small bowl, seasoning with hot pepper sauce to desired heat. Arrange scallions on a tray; dip white ends into sauce. --From "Sunset Vegetable Cook Book" STIR-FRIED LAMB WITH SCALLIONS (6 servings) 2 pounds boned leg of lamb, shank half Lamb marinade: 2 1/2 tablespoons soy sauce 1 1/2 tablespoons rice wine 1 1/2 teaspoons sugar 2 tablespoons water 1 tablespoon cornstarch For cooking: 1 cup peanut, safflower or corn oil 6 cloves garlic, sliced very thinly 5 cups shredded scallion greens or leeks Lamb sauce: 2 tablespoons soy sauce 2 tablespoons rice wine 2 teaspoons clear rice vinegar 1 tablespoon sesame oil

Remove any fat or gristle from the lamb and discard. Cut the meat, across the grain, into slices about 1/8 inch thick. Cut the slices into pieces that are 1 1/2 inches long and 1 inch wide. Mix the lamb marinade ingredients together and pour over lamb. Toss lightly and let marinate for 30 minutes.

Heat a wok, add the peanut oil and heat to 400 degrees. Add half the lamb and stir-fry over high heat, stirring constantly until the lamb changes color. Remove the meat and drain. Reheat the oil and stir-fry the remaining lamb pieces in the same manner. Remove the oil from the wok, reserving 3 tablespoons.

Reheat the wok, add the reserved 3 tablespoons of oil and heat until very hot. Add the garlic and the shredded scallions. Stir-fry over high heat for about 30 seconds. Add the cooked lamb and the combined lamb sauce ingredients. Toss lightly and transfer the mixture to a platter. Serve immediately. --From "Classic Chinese Cuisine," by Nina Simonds CHEESE BOMBAY 8 ounces cream cheese 6 ounces sharp cheddar, grated 1 tablespoon curry powder 2 tablespoons sherry 3 large cloves garlic, minced 1/2 cup mango chutney 2 tablespoons minced scallions

Let cheeses soften to room temperature. Mash cream cheese with a fork. Grate cheddar cheese. Mix cheeses together. Add curry powder, sherry and garlic. Mount on a plate and chill several hours. Frost with chutney and sprinkle with scallions.