Place the fish fillet in a large mixing bowl. Add cornstarch and fish sauce and use a clean hand to toss the fish to let the cornstarch and fish sauce coat the fillet pieces. Set aside while you prepare the other things
Cooking:
Preheat a large heavy-bottom pot with some oil. Add the chopped ginger, garlic, shallots and saute over medium heat for about 2-3 minutes. Add chicken stock and bring to a boil. Add celery if using (I didn't use in the video).Then lower the heat and let it simmer for about 15 minutes until the celery are soft. You can prepare ahead up to this point
When ready to serve the fish soup, bring the soup back to a gentle simmer and add the tomatoes, and the fish fillet. The fish is easily flaked if it's cooked through, if not, let it cook a bit longer. Do not overcook the fish. Have a taste and add more fish sauce to your taste if needed. Remove from the heat
Ladle soup into serving bowl (family style) or individual serving bowl, and garnish with cilantro and green onion just before serving and a squeeze of lime juice if you like