If you've ever driven down El Camino Real in Palo Alto, you might have wondered about a large sign with "TOFU HOUSE" written in teal letters. With the abundance of tofu dishes available today, from Chinese mapo to all sorts of tofu salads, it can be hard to know exactly what to expect.

Despite its pedestrian storefront, So Gong Dong Tofu House is immensely popular and one of the only restaurants in the area specializing in authentic Korean sundubu-jjigae, or soft tofu soup.

Sundubu-jjigae is a staple of Korean cuisine and known as a comfort food. "Sundubu" translates to "pure tofu" and refers to curdled, extra-soft tofu, while "jjigae" means "stew." The soup is typically made with meat or seafood; vegetables like zucchini, mushroom and onion; and finished with a generous amount of soft tofu -- all cooked and served in an earthenware pot, which retains the soup's heat.

Joyce Yoon and her husband Sung Yoon opened So Gong Dong Tofu House at 4127 El Camino Real in 2005. Although the restaurant began small with about five types of tofu soups, they now serve almost 20 varieties, from the traditional -- such as seafood tofu soup -- to more creative adaptations like cheese tofu soup, all added in response to customer suggestions over the years.

Yoon, a soft-spoken, middle-aged Korean native who lives in Santa Clara, guards her recipe for the tofu soup, insisting it is a "secret," but said the key is fresh ingredients. She switched to organic, non-GMO tofu several years ago.

"We tried tofu from many different companies -- the one we use now is the most tasty and savory," she said in Korean. "It's delicious. The texture is pleasing."

Customers can choose from five spice levels for their tofu soup: not spicy, mild, medium, hot or extra hot. While the mild flavor emphasizes the taste of the tofu, the spicy options bring out the signature bold, rich flavor of the soup sauce, typically made with a gochujang (chili paste) base.

After ordering, diners are served a number of "banchan," or small side dishes such as kimchi, pickled cucumbers, seasoned mung bean sprouts and egg rolls (which are not fried, but made into an omelet, then rolled on a frying pan and cut into bite-size pieces), served in traditional metal bowls.

Then the tofu soup arrives, still boiling in the pot, with steaming japgokbap (purple-colored multigrain rice) and a raw egg to crack into the soup. This should be done while the soup is still boiling. Bury the egg promptly under the tofu so it can cook.

Although the restaurant mainly specializes in tofu dishes, it offers a selection of other Korean foods, including bibimbap (rice, vegetables, meat and gochujang also typically served in a stone bowl), marinated and barbecued meats and naengmyeon (cold buckwheat noodles with egg and cucumber).

The menu also has a lengthy vegetarian section.

"In the past, the vegetarian dishes were mixed in with other items in the menu, but we made it its own section to make things easier," Yoon said in Korean, noting that the change seems to have attracted more customers. "Korean cuisine offers a lot of vegetable-based dishes ... especially with the influx of side dishes that come free of charge -- they're all made of different types of vegetables and great for vegetarians."

The owners gradually expanded the menu, Yoon said, when Korean customers came in searching for their homeland favorites or when customers returned from trips to Korea, eager to have certain foods again at her restaurant. She emphasized, however, that she deliberately limits the size of the menu, which is translated from Korean into both English and Chinese.

"We don't have a diverse menu. Compared to other Korean restaurants, it's rather simple," Yoon said. "It's a good way to keep the ingredients fresh."

The restaurant receives ingredient deliveries up to five times a week, so nothing is ever sitting in the kitchen for too long. Yoon said she visits Korea to compare local tofu restaurants to her own. She also travels to Los Angeles, where far more Korean goods are available due to its enormous Korean population (the largest enclave outside of Korea) to handpick dishes for her restaurant.

Yoon described the restaurant as her "baby," having watched it grow since it only had a handful of customers. On any given night, there can be up to 100 people dining at So Gong Dong Tofu House.

While four other So Gong Dong Tofu House restaurants exist in the Bay Area, all owned by other members of her extended family, she and her husband only own the Palo Alto restaurant.

Yoon said she doesn't plan to open any more restaurants in order to keep her focus and energy on this one. Over the last 13 years, she's come to know many customers on a first-name basis and is accustomed to running into them at local grocery stores.

"Some of the kids who used to eat here are already in college. They visit the restaurant during their breaks. ... It's already been that long," she said. "When regular customers stop coming, I get worried and when their children come in I ask how their parents are doing."

Yoon, who immigrated to the Bay Area in the late 1990s, said it's been rewarding to introduce Midpeninsula diners to Korean food and culture.

"They come to the restaurant and share their experience with Korea and we share our knowledge about Korea," she said. "It's really gratifying."