It may seem far-fetched to envision a nationwide chain of Vietnamese fast-food restaurants, but the rapid expansion of Pho Hoa -- whose thirteenth national outlet recently opened in Falls Church -- makes that a tantalizing possibility. With eight places throughout California, the chain claims that state as its home base, but Pho Hoas also are in Seattle, New Orleans, Montreal, and Toronto, and now Falls Church. Depending on thc size of the Vietnamese communities in Baltimore, Philadelphia, Manhattan, and Boston, expansion up the eastern seaboard is a reasonable expectation. After that, the heartland would beckon, and, with a Pho Hoa of its very own, everything really would be up-to-date in Kansas City.

There are no rice-paper-wrapped spring rolls on this menu, no skewers of grilled pork atop noodles, no chicken curry, no steamed fish: You go to Pho Hoa when you want to make an entire meal of either a large or an extra-large bowl of the Vietnamese national soup called pho.

Based on a long-simmered beet stock that is delicately seasoned with star anise and garnished with a generous tangle of noodles, bean sprouts, fresh coriander, paper-thin slices of fresh green or red chilies, Asian mint, and the diner's choice of different cuts of beef, pho is one of the world's great soups. The early opening hours of Vietnamese soup kitchens are rooted in the tradition of pho's origins as a specialty of outdoor-market food stalls, which provide workers and merchants with a fortifying beef-noodle breakfasts and hungry shoppers with a hearty lunch or an afternoon snack that would tide them over until a late dinner. And because the preparation of an authentic pho requires an inordinate expenditure of time and fuel -- in order to extract the full flavor of its meat and bones, the stock must be simmered at least eight hours, and additional time is required to cook the brisket, tendons. and optional tripe that serve as garnishes to the completed soup -- Vietnamese home cooks traditionally have left the work to the professional in the food stall or the restaurant cook.

Pho Hoa and its Vietnamese competitors, both current and those sure to follow, offer what we think is this area's best dining bargain: For less than the price saloons charge for a hamburger, a bowl of pho provides a delicious one dish meal that satisfies the largest of appetites and is nutritionally more sensible than a half-pound of ground beef. The regular portion ($3.75) holds enough that in other restaurants it could serve four as a first course; the large bowl ($4.45) tests the appetite of the most confirmed pho fanatic. But more than merely satisfying hunger. we find that enjoying a bowl of pho produces a smug feeling of having partaken ot something as healthy as it is delicious.

Whatever the diner's choice of garnishes -- at Pho Hoa, they can be as simple as tasty meatballs or paper-thin slices of steak, which, added raw to the boiling broth, cook to a perfect "medium" by the time the bowl reaches the table, or as varied as a grand mixture of eye-of-round, well-done flank, fat brisket, beef tendon, and tripe -- the basic pho starts as a bowl of light-bodied, rich-flavored broth covering a seemingly interminable number of long, soft, white noodles. Accompanying the bowl is an oval side dish, piled high with bean sprouts and topped with a few sprigs of fresh coriander and Asian mint, precariously balanced on the edge are wedges of lemon and some sliced green chili. The diner squeezes the lemon into the broth and adds the bean sprouts, herbs, and chili to taste.

Pho Hoa, 6793A Wilson Boulevard, Falls Church; 534-5501. Open seven days a week 9 AM to 9 PM.