Step 1

Heat oil in a medium pot over medium. Add shallots, garlic, lemongrass, and chiles and cook, stirring occasionally, until aromatics start to soften but have not taken on any color, about 5 minutes. Add tomatoes and cook, smashing and stirring, until tomatoes have burst and fallen apart and mixture is slightly thickened, about 5 minutes.

Step 2

Pour 5 cups cold water into pot and bring to a boil. Reduce heat to medi um-low so broth is at a bare simmer.

Step 3

Add fish to pot; season lightly with salt (you’ll finish soup with fish sauce). Cook until fish is opaque and cooked through, about 4 minutes. Remove from heat; add lime juice and fish sauce. Taste and season with more salt and/or fish sauce if needed. Stir in herbs and ladle into bowls.