One of the most jealously guarded Soviet secrets in this city of intrigue is that the Russian Tea Room - mecca for balletomanes and music lovers - is not really Russian after all. Not only is there not one native of the U.S.S.R. on the restaurant payroll, but the man who really runs the shows is a former cane cutter from Puerto Rico. Sixto Gonzalez.

Not that the Kremlin has clamped the lid on intelligence that this affable non-Soviet has been preparing the pirojoki and blini at the restaurant for 23 years. It just never occured to anyone to mention that the best Russian chef in the Big Apple was raised on rice and beans.

The Tea Room's menu has been a favorite with regulars for over half a century, so no one checks the birth certificate of the chef. Celebrlities and tourists alike flock to the pink and red glitter and polished samovars of a bygone era, taking the chicken tabaka and the nalistnike as they find it.

Young Sixto entered this glamorous atmosphere back in the mid-1950s as an immigrant dishwasher, much like hils contemporary Herman Badillo, who soon parlayed his own chances into a seat in Congress. While thw polirician may be more visble and make more money, Sixto has his own constituency of thousands of satisfied diners, as well as greater security.

Today, at an annual salary of $22,000, he can look back with nostalgia on the rough days when he sailed to the mainland with his young bride, Haydee Cardona, from the little coastal town of Aguada to begin the long climb toward realizing the American dream - Russian style.

"Sixto has probably prepared more dinners for the beautiful people than any other chef around," says restaurant manager Gregory Camalucci.

In any given month, he may prepare dinners for Jackie Onassis, Liv Ullman, Telly Savalas, Sidney Poitier or Benny Goodman. "He's the one who deserves the kudos, although I'm the one who hears them," says Camalucci modestly.

Most Puerto Rican celebrities who frequent the RTR are no more aware of Sixto's existence than the rest, but one Broadway star, Raul Julia, did go back to the kitchen to shake his compatriot's hand, and another - bandleader Johnny Pacheco - enjoyed a special dish of garlic veal the chef whipped up for him. Jose Ferrer, who has had the same front table for 20 years, has never seen the man who feeds him, nor has Rita Moreno, who drops in frequently.

One reason for this is that celebrities and executive chefs keep very different hours. Sixto begins preparations each day at 6:45 a.m., and turns the kitchen over to a night crew at 3:30 every afternoon. When the last Russian chef left three years ago, Sixto was the night chef, charged with carrying out the day chef's plans. It was then that the mantle of responsibility fell to the first non-Russian.

"I have worked with many chefs," Sixto says modestly, "and I have learned to cook hundreds of Russian dishes. The only thing I cannot do is speak Russain."

He doesn't need to anymore, with a staff of Puerto Ricans, Greeks, Austrians, Japanese, Cubans, Hungarians and others. Easy-going Sixto gets along with all of them, acting as mediator when border tensions erupt.

"This could only happen in the Russian Tea Room," he quips, "certainly not in Russia."

The lack of the real thing in the kitchen doesn't seem to worry owner Faith Stewart-Gordon, a regal blond, former actress from Spartansburg, S.C. While Sixto gives her a lot of credit for helping Puerto Ricans advance, Stewart-Gordon claims she wasn't doing anything out of the ordinary.

"He's a wonderful preparation man," she says with characteristic understatement.

Perhaps a French chef raised in a hotel kitchen would be more original with the seasoning, or an Italian chef more creative with sauces handed down for generations, the manager conceded. But no one smooths the waters better than non-temperamental Sixto and in this high pressure, behind-the-scenes operation diplomats rate higher than prima donnas.

In turn, Sixto is pleased with his new kitchen, part of the vast remodeling and expansion the 195-seat restaurnat underwent this year. It's got a steamkettle, fryer, convection steamer and a special bake shop, all of which help make his days a little smoother.

"It's fantastic," he beams, "and it's much easier to clean."

At home with his wife in the Bronx, he tries out the Italian, Hungarian, Chinese and Janpanese dishes he learns from his coleagues at the Tea Room. Although Haydee, who works in a dress factory, is an excellent cook, Sixto likes to pitch in when traditional Puerto Rican cuisine is on the menu. He has a weakness for rice and beans and arroz con pollo, chicken with rice.

All of which helps to pile on the pounds. At work, Sixto tastes a lot of sauces, and is constantly on guard to keep his weight down. But, at 5 feet 6 inches and 190 pounds, it appears to be an uphill fight.

"I never sit down to eat, and I try so hard to lose," he moaned, sampling a special Russian pastry.

Whenever he gets a few days off, Sixto goes fishing with relatives in Massachusetts. The rest of the time, he's simple family man who seldom strays from home.

Although his two children often visit him on the job, his wife has never been in the restaurant. When he does take off his apron and sits down at a pink-clothed table for an interview he wears a leather wind-breaker and keeps one eye on the ketchen.

All in all, Sixto considers himself "very lucky" and "very proud."

One day, he adds with a faraway look, he would like to own a little restaurant in a small town out of the glare of the bright lights, preferably in Puerto Rico.

But at 48, it is much too soon for Sixto to leave the helm of his famous Russian ship.