(FORTUNE Small Business) – It sounds almost like something out of an Amy Tan novel, but with a Vietnamese flavor. For decades the An family was part of Saigon royalty, but in 1975, when the city fell to communists, the Ans were forced to flee. They came to San Francisco with nothing but the recipes their family's personal chefs used to prepare. Scrambling to make ends meet, matriarch Diana An, her daughter Helene, and five granddaughters went to work, turning the cuisine they grew up with into a flourishing, 150-seat restaurant called Thanh Long. Their signature dish is a platter-sized Dungeness crab fresh from the Pacific, drenched in butter and secret spices. One crab makes a meal, and they come pre-split, so you don't create much of a mess (even though you might want to). It's not unusual for the Ans to sell 200 crabs on weeknights and 500 on Fridays and Saturdays. An army of efficient staffers whisk them continually from the kitchen, bright red and vaguely menacing on their platters. Most diners also order the heaping plates of creamy garlic noodles, with or without grilled tiger prawns on top.

At Thanh Long the crabs have legs, but the recipes don't. To prevent employees from taking the special family dishes to competing restaurants, Thanh Long has a walled-off secret room within the main kitchen, where only Ans can enter. The family has since opened another, more upscale restaurant in San Francisco called Crustacean, and a third in Beverly Hills, but Thanh Long remains the most authentic venue. "The restaurant that saved us should always be standing," says granddaughter Hannah An. "It's the roots of our family." 4101 Judah St., San Francisco; 415-665-1146. Dinner only; closed Mondays. Reservations a must.

--Julie Sloane