Pick the leaves off the celery stalk and reserve for later use. Cut the celery stalk into 2-inch lengths. In a mortar, combine the celery, chile, spring onion (white part only), and garlic then slightly pound into a coarse paste. Pour the fish stock into a large pot and bring to the boil.
Add the cod cutlets, sugar, and fish sauce, and then bring back to the boil skimming out any impurities. Add the celery stalk-chile mixture.
Reduce the heat to a low simmer and cook for 10 minutes. Now add the tomatoes, celery leaves, and spring onions (green parts) and cook for another 5 minutes. Take the pot off the heat and transfer to a large bowl, then add the bean sprouts, and lime juice and stir.
Serve the soup with the jasmine rice and side dipping sauce of sliced chile and fish sauce.