Canh Chua Cá Thì Là (Vietnamese Fish Soup With Tomato and Dill)

Canh chua cá thì là, a tomato fish soup fragrant with dill, is one of countless renditions of canh chua (literally translated as “sour soup”), a category of Vietnamese soups generally consisting of fish or seafood simmered in a tart broth. This version is light, delicate, and refreshing, and characterized by the sweetness of the fish mingling with a mild tangy tomato broth and the fragrant, herbaceous note of the dill.

Regardless of the season, there’s always a soup in a Vietnamese meal. Collectively known as “canh,” these soups bring balance to bold and assertive kho (braises and stews), cleanse the palate, and can help moisten the rice for smoother ingestion. Most everyday canh are a humble fare, where vegetables are cooked in water or a quick stock that gets its savory depth from either ground pork, dried shrimp, or mushrooms. Because of Vietnam’s long coastline, and the extensive network of rivers, ponds, and lakes in the Mekong and Red River Deltas, canh chua is also a way to celebrate the abundance of aquatic life. People rely on the acidic broth to temper the earthy or muddy taste that can be found in freshwater fish (caused by geosmin, a chemical naturally produced by algae and microorganisms in freshwater).

“One thing that people might miss sometimes when talking about Vietnamese food is the [importance of] sourness,” says Linh Khanh Trinh, a Vietnamese culinary historian pursuing her PhD at the University of Michigan. “There are several ingredients that can go into the sour flavor: vinegar, tamarind, starfruit, tomatoes, and fermented rice.” In the case of homestyle canh, which are less commonly found on restaurant menus, the source of the sourness varies depending on the family and the region. Each of the acidic ingredients can be used on its own as a source of sourness, but I've also seen some used in combination (like tamarind and pineapple).

In Southern Vietnam, where I’m from, a bowl of canh chua often brims with okra, bean sprouts, and taro stems (a distant cousin of actual taro); its broth is decidedly sweet and sour from tamarind and a liberal amount of sugar. In contrast, Trinh, who was born and raised in Hanoi, grew up with canh chua cá, a simpler rendition with fewer ingredients, its mellow tang derived from tomatoes, mẻ (fermented rice), or fruits like sấu (also known as the Indochina dragonplum). Elsewhere in central Vietnam, the soup’s tartness and mild astringency might come from pickled mustard greens, pineapple, or unripened bananas. There’s also a bit of heat from the addition of chiles.

While canh chua cá thì là is generally made with freshwater fish, this recipe is flexible and can accommodate different varieties (fatty and oily fish like salmon will have a much stronger flavor than snapper or sea bass). A whole fish cut into steaks is commonly used, though Trinh’s family exclusively makes their soup with a freshwater fish head because of its sweetness and richness.

I start the soup by giving the fish a quick sear to lessen any muddy flavor, a technique I learned from both Trinh and Andrea Nguyen’s Into the Vietnamese Kitchen. From my testing, I found that searing carp and tilapia significantly removed their earthy flavor, but for seabass and salmon the effect was negligible (they only had some mild brininess to begin with). This step also gives the fish a sweet toasty note and forms a crust on the surface that can help prevent breakage in the soup, so don’t skip it if you’re not pressed for time.

For the tomatoes, I dice a third and sauté them right after the shallots to concentrate their flavor for the soup base, and cut the remaining tomatoes into wedges to add near the end of the cooking, which helps to maintain their brightness. Depending on how tart your tomatoes are, you may want to add a splash of rice vinegar or a squeeze of lime juice to adjust the acidity. The broth won’t have a pronounced sourness, but should have a mild tang with some subtle sweetness.  

To serve, simply ladle the broth into a large serving bowl and transfer the fish to a sharing plate so diners can easily pick the flesh with their chopsticks. For a full Vietnamese home-style meal experience, combine fish sauce and sliced bird’s-eye chiles to make a dipping sauce for the fish pieces, which can also be used to season the soup in your own bowl if you find it needs more kick.