Open for dinner Monday through Thursday 6 p.m. to 10:30 p.m., Friday, Saturday 6 p.m. to 11:30 p.m., Sunday 6 p.m. to 10:30 p.m. AE, CB, DC, MC, V. Reservations suggested. Prices: Fixed-price dinner $15.

The restaurant 2091/2 has spawned a commoner. Named

after the Duchess of Windsor, who was the vehicle for

transforming Edward VIII from king to duke, Mrs.

Simpson's is a branch of that delicious little gem of a

restaurant, 2091/2, but lacks its regal elegance.

You wouldn't know it to look at it. Mrs. Simpson's matches its Capitol Hill relative in its fashionable dress, the walls in herringbone tweed and the bare wood tables softly lit by brass hurricane lamps. The jewels that set this restaurant apart from the masses are the royal memorabilia--plates, snuff boxes, photographs and newspapers--in vitrines, on the walls and even in the bathrooms. Predictably, they are fascinating. Surprisingly, they are also decorative.

But what does one eat in such a museum? One starts auspiciously, with a tiny plate of canap,es to nibble as you read the menu, perhaps very good liver mousse on toasted french bread or anchovy spread on black bread. The menu itself is short and fixed- price, in the mode of 2091/2. You can start with a green salad or with vegetable soup afloat with cheese- topped bread in the manner of onion soup gratin,ee. The soup is one of the best dishes on the menu, chock full of homey vegetables and meaty bones, though it has been repeatedly too salty. There is also an interesting warm salad of bright and crisp green beans tossed with chicken cracklings in a vinaigrette. And the fashionable celery root remoulade is this time mixed with cauliflower.

Beginnings are best. As for main courses, there are eight on the menu and usually a special or two. The ideas are clever and the flavors interesting, but even the best of the dishes need some work. Almond-fried shrimp are butterflied plump ones in a crunchy crust of finely chopped almonds, fried just enough to leave them juicy. But their chowchow accompaniment doesn't keep you from missing something zesty to dip them in. The successful main dishes at Mrs. Simpson's were the duck with lemon and oregano, tangy and crisp-skinned, with moist and fresh meat (but too much remaining fat); a special of grilled chicken, also perky in flavor and not overcooked, though the meat was tough and its wine sauce was just an anonymous wash that did little but wet the chicken. Lamb chops were also grilled, nicely charred and pink inside, a neat trick since they were quite thin. And fish was cooked accurately, though it was not as fresh as it ought to be, a fact that was not hidden by its overpeppering. The out-and-out disaster, though, has been "today's pasta with three cheeses," which has been repeatedly aswim in a vapid and watery sauce that tasted hardly of one cheese.

Besides, when you order the pasta you miss the best part of Mrs. Simpson's--the potatoes au gratin, crusty and golden and cheese-laden.

Other accompaniments fall flat. Rolls have sometimes been chewy, and while the waiter's offering of pommery mustard and b,earnaise to accompany main dishes is a nice touch, the b,earnaise has been so bland as to be indistinguishable in taste from the hollandaise or even from whipped butter. The accompaniments also include a pleasant surprise of a relish, one day raw cranberry, another day a vinegary eggplant.

What the waiter ought to be offering, though, is your wine. The service has a certain calm and dignity that befits a museum to British royalty, but even the British know better than not to bring your wine until your entree is half consumed, then to explain that they hadn't brought it before because they didn't know when you wanted it. The wine list is well chosen, certainly worthy of better service.

Whatever the snags, hang on until dessert. There are only three choices, but they are delightful ones: a fresh and cloud- like orange mousse, prunes deliciously and alcoholically marinated and topped with whipped cream and a dense--if slightly dry--chocolate mousse cake.

A restaurant of moderate size, moderate price and moderate accomplishment is Mrs. Simpson's. If it is to succeed as a neighborhood restaurant, it will need a more varied menu; four appetizers and three desserts can quickly grow boring no matter how good they are. If it is to draw a metropolitan following, it will have to perk up the details of kitchen preparation and dining room service. In all, though, even with its flaws it is a nice place to visit.