345-6101 Open for lunch Monday through Friday 11:30 a.m. to 2:30 p.m.; for dinner Monday through Friday 5:00 p.m. to 10:30 p.m. and Saturday and Sunday 4:30 p.m. to 10:30 p.m. AE, MC, V. Reservations suggested on weekends. Prices: Dinner appetizers $2.25 to $3.25, main dishes $6.95 to $14.50. Full dinner with beer, tax and tip about $15 to $20 a person. Beer only.

You don't have to be an old American to cook the new American cuisine. At Chef's Secret, a Thai and an Austrian, working side by side in the kitchen, are taking the best and freshest of local raw material--mainly seafood--and turning out some superlative dishes.

What these chefs do is not just a pinch of Thai and a pinch of "continental," but something different from both. They understand their ingredients, so fish is carefully cooked to resist the fork a little. And they understand restraint and good judgment, so sauces are tailored to complement rather than overpower. Chef's Secret, which opened in August, would be a treat anywhere, but it's a special treat in Greenbelt, an area where restaurants in past years have tended toward neon signs that say "cocktails" before "food," and where prepackaged, premerchandised "theme" restaurants now proliferate amid the fast-food parlors.

From the outside, Chef's Secret is undistinguished, the latest of several restaurants to occupy a nondescript little building next to a gas station on Greenbelt Road. But step inside. The dining room shows the same quiet grace and good sense as the food. On the tables there are white linens, fresh flowers (orchids among them) and delicate china; and on the walls, paintings from a local gallery. This is the kind of place--soft, warm candle-lit --that seduces you to settle in for a long, comfortable evening. The service, on the other hand, can veer crazily from smooth and quick one night to absent-minded and incredibly slow the next.

Appetizer decisions: the problem with ordering the good clams casino is that you may miss the even better mussels mariniere, bathing in an oil-wine-garlic-parsley sauce that is wonderfully soppable with the good french bread. (The mussels poulette don't have nearly the zip.) But mussels may cause you to miss the Scottish salmon, a generous portion, with a smokey softness that makes even the best Nova lox seem overstated. And then there's the odd but good seafood au gratin, a big crock of extra-thickened New England-style chowder crammed with firm bits of clam, shrimp and scallop, and topped with melted cheese like some nautical onion soup.

Ordinarily, if a predominantly seafood menu listed a few pasta dishes, you'd do well to ignore them. But general rules don't apply at Chef's Secret. Pastas here--nicely chewy and beautifully sauced--would credit any northern Italian restaurant, and they're available in half-portions as appetizers. Fettucine alfredo, so often a close cousin to wallpaper glue, is a paragon of lightness. Even better are the feathery canneloni and the agnolotti alla panna, pasta envelopes stuffed with spinach and served with a delicate cream sauce.

Pity the poor bluefish. The fatty layer under its skin oxidizes quickly if the fish is not perfectly fresh, so it is often unjustly accused of having a "strong" flavor. Help exonerate this maligned species by ordering bluefish posilippo, when it's available, and notice how delightfully sweet and unfishy even the fat is. This is a pearly slab of fish, beautifully broiled, surrounded by clams and mussels and served with a bright, slightly hot tomato sauce with just the right robustness for such a robust fish.

There are delicate sauces for delicate fish, too. The light pink Norway salmon, another occasional special, is complemented by just a little subtle, buttery topping. And the firm, juicy swordfish--the ocean's answer to prime rib--has a dark sauce that accents its succulence.

We asked a much-traveled friend about his experience with bouillabaisse. "I've had it so often in bad places," he joked, "that I know the two basic criteria: a layer of oil on top and a layer of grit on the bottom." You get no excess oil at Chef's Secret--and not a grain of grit. What you do get is lobster, fish, shrimp, mussels and scallops, all fresh, firm and glistening in a lightly garlicked broth that's enlivened by what tastes like turmeric. A jewel.

The same ingredients, along with crunchy green pepper and onion, appear in an immense and magnificent seafood brochette, broiled with what seems to be a sprinkling of good, hot paprika, and with a minced garlic sauce on the side that proclaims the Thai chef's roots. If you fancy more unalloyed Thai-style cooking, order the beautiful jumbo shrimp with garlic sauce. A similarly garlicked sauce, but with a tomato base, accompanies the good scampi Mediterranean. But that tomato sauce is also used in a more spectacular dish, seafood en papilote, a foil dome in which are steamed striped bass, shrimp, mussels and scallops--a top-notch match for the bouillabaisse or the brochette. Forget the "Fish 'n Chips England Style," even though you may be intrigued at what a Thai and an Austrian would do with this dish. It tastes depressingly like what they serve in the cheap fish houses. Veal dishes are good--certainly adequate if there's a no-fish person in your party--but the veal is no match for the seafood.

Entrees come with vegetables that are generally very good, with potato or rice, and with coleslaw (tasteless) or salad (excellent). And note that the occasional appetizer special called "spinach, mushroom and bacon" is actually an excellent spinach salad at a bargain price.

Desserts, made in-house, are generally acceptable, if unspectacular: ordinary cakes enhanced by better-than-ordinary toppings, a credible but oversweetened mousse, and, perhaps simplest and best of all, fresh berries with sabayon sauce.

We intentionally saved some of the best news for last: the remarkably low prices make this one of the best dining-out values around town. There's a bit of a drawback, for now, too: Chef's Secret doesn't yet have a license for wine or hard liquor, so beer is the only alcoholic beverage presently available.

A final note: For all the unpredictability of the service, Chef's Secret is a local jewel. If business is good, if they learn to handle a full house without forgetting things in the kitchen, and if the quality doesn't take a nose dive or the prices a big leap, it should prove a durable treasure among suburban restaurants. But treasures deserve some nurturing; a relatively new little place like this needs protection from being suddenly swamped. So tuck this review in a drawer with a note to visit in a few weeks, and preferably on a week night. It should be worth the wait.