La Brasserie, 239 Massachusetts Ave. NE, 546-9154.

Hours: Monday through Saturday, 11:30 a.m. to 11 p.m.; Sunday, 11:30 a.m. to 10 p.m. Prices: Dinner hors d'oeuvres, $3.50-7; dinner entrees, $12-21. Cards: American Express, Diner's Club, MasterCard, Visa.

Now that the '85 spring has popped its cork, a host of putative patisseries will splay across the sidewalks of Washington, pandering to the yuppie palate. For most of them, the ambiguous ambiance (gourmet delights by traffic light) will be appropriate to the fare.

But La Brasserie, which well into its third administration qualifies for some Capitol Hill seniority, describes itself as a gourmand's haven, and it takes its food seriously. (It takes its prices seriously, too.) With its cavalier attitude toward dress codes and equally democratic outlook toward reservations, La Brasserie remains a favorite for special dinners out, and especially for the extra dessert you wouldn't allow yourself on ordinary occasions.

The food seems especially appealing in warm weather: The standard onion soup gratinee gives way to daily doses of cream of watercress or thinnish cucumber soup; the pate and salads are augmented by platters of fresh asparagus and a luxurious, home-cured Norwegian salmon that can set you back $10 for the first course.

The regular terrine of salmon and green beans (an intriguing duet of rich smokiness and nasal tang) goes plate-a-plate with a tri-color seafood terrine banded with deep green leaves and served with pungent diced tomato. One could easily make a meal of assorted appetizers here, especially with a bottle of wine and the kitchen's fine crusty bread.

Among the regular items are a classic pot au feu (a chicken stew far more delicious than its New England descendant would lead you to think) and a slab of liver with crusty potatoes. On the ritzier side is a plate of duck cooked rare and sliced thin; garlicky, aromatic mussels; and a filet that at $20 dares to be grandiose.

The weekend gem -- usually served on Saturday, but not invariably; phone ahead -- is bouillabaisse served in a manageable soup bowl, rather than a tureen, but with a careful mix of goodies.

The specials are hard to resist, however. On a given day, they may include soft-shell crabs, mercifully free of flour, sauteed with garlic and a touch of tomato; squab in a syrupy but not-too-sweet cassis sauce; and a cooled chicken lobster, tail and claw meat carefully removed and reassembled on the plate.

(The lobster, incidentally, was fashionably decorated -- the red theme was carried out by squibbles of tomato paste around the edge of the plate -- but the mayonnaise needed a bite of herbs. And the only side dish provided was a flawlessly smooth but dangerously pallid egg custard, which offered no contrast to the seafood. Similarly, the mixed vegetables that come with other dishes are pleasantly crisp but plain.)

Desserts, which have always been among La Brasserie's strongest suits (the bittersweet chocolate mousse, studded with strawberries, is a byword), remain to haunt the indulgent. Each day there is a list of fresh fruit tarts, a splendidly caramelized creme brulee, a white chocolate mousse (respectable but undistinguished) and a chocolate mousse cake that can be the blood-sugar equivalent of Stupefyin' Jones. But recently, a new temptation has hit the sidewalks of La Brasserie: strawberries or raspberries, run under the broiler in a caramelizing sauce with Kahlua. It's to diet for.