Put tomatoes, onion, garlic, chili pepper and salt together, and crush them into a paste using a mortar. You could instead use a blender or food processor.
Coat a saucepan with the oil, over medium heat, then add the fish and the paste you just made. Stir, then add the broth to the pan.
Put the scallions into the pan. Add more broth if wanted.
Reduce heat and let the soup simmer about 15 minutes. Add fish sauce and serve.