Place swordfish cubes into a bowl and season with 2 tbsp of fish sauce and then set aside.
Heat oil in a large pan on high heat, then add carrots and scallions, toss until flavors begin to release and then cover and lower heat to a simmer.
Allow the carrots to steam and par cook.
Add celery and chillies, toss and cover and allow to simmer once more for approx. 5 minutes.
Add lime zest, the balance of the fish sauce, rice vinegar and 4 cups of water. Cover and let simmer for 10 minutes.
Add balance of water and then drop in swordfish pieces carefully.
Cover again until swordfish is cooked through (approx. 5-8 mins.)
Serve and garnish each bowl with two slices of radish and a pinch of lime zest.