Open Monday through Friday, 11:30 a.m. to 3 p.m. and 5 to 11 p.m.; Saturday, 5 to 11 p.m. BA, MC, AE. Reservations.

Food: Good Mexican food, a walk on the mild side.

Style: Tiled, dark and handsome.

Price: A well-fed couple can get change from $25.

HOW, YOU MAY ASK, can a go od Mexican restaurant serves an indifferent enchilada? There is more to la vida, I answer, than tortillas. La Fonda has lots of assets, but least among them is its tortillas. The chips taste little better than Doritos (which is not to deny their insidious appeal). Tacos are meaty and tasty, but shells lack a proper earthy quality. And, while the enchilada suiza draws favor for its piquant sour-cream topping, its cornmeal underpinnings have tasted dried out.

Why, you then ask, would one bother to go to La Fonda? Obviously, I respond, for its beef dishes, for its mole, for its vegetables and rice and its shadowy Mexican beauty. It is not the Mexican restaurant of our dreams, but it will hold us until that comes along.

And people who don't have their dreams invaded by fiery Mexican food should La Fonda anyway, because it illustrates ont of the best kept secrets in the culinary world, that Mexican cooks have an understanding of subtlety.

La Fonda's two dining rooms are among the more charming places to linger over a dark or light Mexican beer. The well-spaced tables allow privacy, especially since even at lunch it is dark. Mexican tiles, one of the most attractive decorating devices in existence, are put to good use on La Fonda's floors and walls. And dinner takes on a party air from the colorful handwoven tablecloths and the candles in hurricane lamps. La Fonda is fetching pretty. Lunch tends to be nosiy, clattery, Latin; at dinner it falls into a romantic hush.

In either cases, the mood is festive, the waiters are personable, and, depending on the day, service has ranged from brisk to manana.

The meal starts with a basket of tortilla chips and a dipping sauce or two. The thin red pureed sauce is merely wishy-washy, but the chunky sauce is a bright combination of chopped fresh tomatoes, onion, green chiles and cilantro (fresh coriander), turning chips into a base for a miniature salad. Right away is a good time to start your beer. There are also elaborate tequila drinks, but be forewarned that the sweet ones are very sweet, as is the sangria - which otherwise is quite nice and, contrary to the menu, can be ordered in a small pitcher for $2.95.

Appetizers, at least six of them, are usual variations on a tortilla theme: tacos, tostadas with chicken or beans, nachos, and thinly rolled taquitos. While the tacos are not very spicy, they are filled with lush crunchy salad and plenty of beef pr chicken, scented with oregano. I prefer the taquitos, topped as they are with avocado and what tastes like homemade sour cream. Empanaditas, tiny fried turnovers with a mild ground-beef filling, are likable, though unnecessarily greasy. For a lighter appetizer, try the very tart, peppery ceviche, chopped fish and shrimp in lime juice with onions, peppers and cilantro. Mexican cactus salad makes a pretty red and green appetizer, but has more frozen Italian green beans than cactus, and more lemon than is needs.

Two main dishes stand out as La Fonda. The carne asada is a marinated flank steak, loudly announcing the presence of vinegar and fragrant Spanish paprika, charcoal-grilled as rare as you ask for it. It is a chewy piece of meat - which does not mean that it is tough - and a scintillating dish for flahhing summer appetites. The guacamole that accompanies it is light and fesh, a nice contrast to the meat's piquancy. Along more robust lines, the chicken with mole, available only at dinner, does credit to one of Mexico's great dishes. Its sauce is nearly black, hot but not blistering, presenting waves of flavor - vinegar tartness, chocolate bitterness, the grainy, slightly sweet undertone of many spices pounded to a paste. With the chicken and the carne asada comes saffron rice of distinction, well flavored with chicken stock, each separate and chewy, delicately tinted orange-gold.

The mole sauce also mates well with chorizo-potato enchiladas, or you can have enchiladas with chicken, white cheese or Swiss-style with chicken, green sauce and sour cream. Again, don't expect fiery seasonings, but do expect the statisfaction of deftly cooked meats and vegetables. The same can be said for chiles rellenos: fresh, crisply battered, with a nice tomato puree on top, but definitely low-key. Least spicy of all is acarne de torito Chihuahuense, which translates into a faintly seasoned big charcoal-grilled steak served with knockout side dishes of zucchini sauteed in oil with onions and tomatoes, and similarly cooked potatoes and carrots with onions. On goes the menu to chile con carne, fish with peppers and onions, shrimp with baby cactus, and chicken with rice. At lunch there are eggs with tortillas and pepppery tomato sauce. If divest yourself of the notion that Mexican food is supposed to leave you gasping, you will find that this is good home cooking.

Portions are large, but the homemade puddings may tempt you further. The flan and nartillas are less rich with egg than I would like, but the flan has delicate texture and is attractively caramelized.The natillas, chewy with coconut, packs a strong liqueur punch, at least at dinner. Take a chance on the coffee; one day it was weak, another day a memorable dark, intense brew.

At lunch, main dishes are $4.25, appetizers around $1.50, desserts $1. Most of the dinner main dishes are $4 or $5, with $8 the highest; appetizers and desserts are $1.25 to $2.With moderate prices and immoderate charms, La Fonda is a comfortable place to wait for the great Mexican restaurant Washington keeps saying we need.