You won't get any ketchup, only mayonnaise, with your french fries at Le Mannequin Pis. The list of beers runs to more than 40 different labels, some of which come swaddled in tissue paper and deliver a seductive, Champagne-like pop as they're uncorked. Breathe in: The air hangs fragrant with the perfume of freshly steamed mussels, woodsy sauteed mushrooms and melting cheeses.

A map reminds us we're in Olney but an hour at the table transports us to Belgium, the source of inspiration for this cozy, 48-seat outpost nestled in a tiny shopping strip off Route 108.

Launched on April 1, Le Mannequin Pis introduces diners to such Flemish traditions as waterzooi, a fish- and vegetable-laced stew made rich with butter and cream, and the whimsy of its 33-year-old muse, Bernard Dehaene. A native of Brussels, where he took up his craft at age 15, the chef has been cooking in this country since 1985, beginning at La Ferme in Chevy Chase; since then, he's added such disparate kitchens as Serbian Crown in Great Falls and Restaurant Nora in Washington to his resume.

"I want this to be a bistro where people come to forget about their problems for a few hours," says Dehaene. That's hardly a challenge in this convivial environment, whose black, yellow and red color scheme echoes that of the Belgian flag and whose cooking draws a veritable United Nations of diners: English may be only one of several languages to compete with background music that pays allegiance to no particular country. And busy weekends may find Harley riders sitting alongside suited denizens of nearby Leisure World, plus families with kids in tow and yupsters on dates.

Where to dive in? Regulars swim straight for the steamed mussels, which can be ordered multiple ways--simply, flavored with white wine, celery and fresh herbs, or nouvelle with the addition of goat cheese or lamb stock--and which show up by the kilo, in dark, double-decker pots. Lift the lid and a cloud of steam bursts forth, offering diners an impromptu facial along with the pleasure of plump, meaty bivalves. Rounding out the picture is a golden haystack of long, thin, crisp and irresistible pommes frites (french fries), better for a dip in the accompanying mayonnaise. Tangy with lemon juice and vinegar, the condiment is based on Dehaene's mother's recipe.

The staff encourages diners to tackle the mussels like the Belgians, who select an empty shell from the mound and use it like a pincer to remove the meat from the remaining specimens--"Belgian chopsticks," one waiter calls them. After the last mussel is knocked back, the shell that served as a scooper is broken in half, while a wish is made, for good luck.

Belgium's cool clime and northern roost reveal themselves in dishes that lean to the straightforward and hearty. At Le Mannequin Pis, that translates into piping-hot cheese croquettes that play the part of breaded fondue, and pork that is stuffed with figs and spinach, then roasted and set out with clove-scented red cabbage. Or tender chunks of beef, arranged in a pyramid, crowned with a dollop of brassy mustard and encircled by a moat of brick-colored, beer-infused sauce. A river of cream and butter appears to have made its way into the waterzooi, but we throw caution to the wind--hardly a drop of liquid remains as we swab clean the bowl with pieces of bread. In all, this is honest fare, appealing in its simplicity.

The restaurant's name? It, too, is linked to tradition. The statue of the little boy that graces the entry of Le Mannequin Pis turns out to be a replica of the famous "pissing boy" fountain near Brussels' Grand Place--an authentic note, right down to the tinkling stream of water that serenades diners.

Le Mannequin Pis, 18064 Georgia Ave. (at Route 108), Olney. Call 301-570- 4800. Open for dinner Tuesday-Saturday, 5-10 p.m.; Sunday, 5-9 p.m. A dinner for two, with drinks, tax and tip, averages $45 a person.