Open for lunch Monday through Friday; for dinner 6 p.m. to 10 p.m. Monday through Saturday. Closed Sun. AE, CB, DC, MC, V. Reservations required at lunch, suggested for dinner. Parking in building. Prices: At lunch sandwiches $4.50 to $6, main courses $5.50 to $8.50; at dinner appetizers $2.75 to $10, main dishes $8.50 to $27.50; average dinner with wine, tax and tip about $35 to $40. Fixed-price evening menu $12.50.

Some restaurants you dine in because they are there; others you dine in because you are there. You're looking for a place to eat near a shopping area or the theater, and there it is: the perfect choice is a restaurant that otherwise might not be on your list.

That's the saving and the downfall of JJ Mellon's. If you work in the neighborhood, are going to National or Ford's Theater or are shopping on F Street, it's as good as you are likely to find nearby. But not enough people are doing that and looking for dinner to keep the restaurant from echoing in nighttime emptiness. And the quality isn't high enough to make JJ Mellon's a destination on its own. Lunch is lively--and the restaurant serves a lot of food for the money at midday. As for dinner, proprietor Joe Malhotra suggested it will probably be two years before Pennsylvania Avenue redevelopment brings enough nightlife to the area. And that gives JJ Mellon's time to get itself in order.

The restaurant has environmental assets: two floors of dining with a spiral stairway in between and "outdoor" tables in the atrium. In the more formal downstairs dining room are velvety gray chairs, banquettes and carpet contrasting with handsome dark auburn wood and minor accents of brass and mirror. It's a Washington Steakhouse in the vein of Joe and Mo's and Gary's: two- fisted portions in a velvet glove.

Ingredients are fair to excellent. There's good lump crab for the crab cakes, endearingly free of breading and grease. The fish steaks are pearly fresh, the shrimp are large, and the beef is cut thick and well trimmed, though not really flavorful enough. The problem is with the cooking and seasoning. If you stick to the usual stark steakhouse-style food, you can do fine. Miniature crab cakes serve as a very good appetizer (and are also made large for a main course); grilled fish is a good bet if you demand it not be overcooked. And while the steaks and chops aren't outstanding, they are big portions of better-than- average New York sirloins at $12.50 for 12 ounces and $16.50 for a pound. Mustard-crumbed rack of lamb is large and reasonably good ($16), though ours was more cooked than we requested. I have found some good soups--chunky and homey beef with vegetable and an overthickened but well-flavored clam chowder--and some disastrous soups--chicken with matzo ball too salty to tempt a second taste, and overthick lobster bisque that was all spice and no depth.

At lunch you can find decent he-man stuff, like a giant mound of shrimp salad in big chunks, flanked by similar-size portions of fresh and blessedly plain coleslaw and nicely tangy potato salad. You can eat a week's supply of protein in the guise of roast beef hash, well browned, crisp and oniony, topped with a poached egg. The lunch menu also lists omelets --at a hefty $5.50 to $7.50--plus homey choices like pot pies and hot beef sandwiches, and naturally the de rigueur burgers and chef's salads.

JJ Mellon's gets into trouble when it tries to turn sophisticated. Crab claws were stringy and tasteless, and fried broccoli was gummed up with heavy batter and sauced with a weird sweet and salty soy combination. The waiter may push for the barbecued swordfish, but I would counter that the strong and vinegary barbecue sauce would do better on a steak or might even serve the fried broccoli, but it overwhelms the fish. And in case you are tired of duck Ma l'orange, Mellon's has duck with grapefruit, but it will send you right back to oranges; the sauce is as sweet as a popsicle, and it turned the wild rice into soup while waterlogging the duck meat and threatening the skin's crispness. Sauces taste amateurish, from the bland and oily hollandaise on some nicely firm asparagus to that dessert-worthy grapefruit sauce. Mellon's does get credit, though, for a delicate and delicious fettucine alfredo at lunch one day; for once the sauce was just creamy enough, just seasoned enough, just thick enough and not overwhelmed with cheese.

The biggest disappointment was the roast beef. What ought to be better in a steakhouse? It was indeed big and thick, with the bone protruding beyond the plate. But it tasted soggy, and reminded us how boring a big pink piece of prime rib could be.

Thank goodness for the steakhouse salads, big juicy m,elanges of greens and onions and tomatoes and olives, or of oranges and walnuts. Your steak or fish comes with no accompaniment more substantial than watercress, but you can fill up on bread: the basket holds several loaves that are interesting but quite sweet, among them a fruit-nut loaf and a toasted nut-anise bread.

The waiter apologized for the wine list, and it was appropriate that he do so. We could find a couple of attractive California and French reds to stand up to the beef, but prices are high and the list is short.

For dessert you'd expect cheesecake, right? Here it comes plain or praline, the heavy and rich kind that tastes delicious for three bites, but the portion is enough for six people. Go easy on it. If you like amaretto, it is whipped into a pretty good mousse; if you like chocolate, you might skip the white- and-dark-chocolate mousse because it doesn't taste much of chocolate.

At JJ Mellon's forget the stone crab craze, the fried vegetable craze, the fresh tuna craze (it was overcooked), the barbecue craze and the roast duck craze. Anything that belongs in a tearoom, that strikes you as cute or sounds gushy you should approach with caution. JJ Mellon's may need those two years before it settles into solid quality, but, given its competition, it does light up a pretty barren restaurant neighborhood. So next time you want to know where to dine before the theater downtown, don't call me, call JJ Mellon.