Heat oil in a medium pot over medium heat. Add shallots, garlic, lemongrass, and chile and cook, stirring occasionally, until aromatics start to soften but before they have browned, about 4-5 minutes.
Add tomatoes and cook, smashing and stirring, until tomatoes have burst and fallen apart and mixture is slightly thickened, about 5 minutes.
Pour 5 cups cold water into pot and bring to a boil. Then reduce heat to medium-low so broth is at a simmer.
Carefully add fish to pot; season lightly with salt. Cook until fish is opaque and cooked through, about 4 minutes.
Remove chowder from heat. Add lime juice and fish sauce. Give a stir and taste. Season with more salt and/or fish sauce if needed.
Ladle chowder into bowls and garnish with herbs, if desired.
Notes
*If you don’t have tomatoes in the house, use a can of San Marzano whole peeled tomatoes. Drain and rough chop the tomatoes.This chowder is perfect for late-season tomatoes you may have from your garden.Since the fish is cut into 1 1/2"-2" pieces it cooks quickly in the broth.Shrimp, clams, and/or mussels can be added in addition to, or instead of, the fish.