1. Mix the cod with 1 tsp of the Thai fish sauce and place in a bamboo steamer or colander. Top with the lime zest and half the chilli. Place over a large pan with a little boiling water, cover and steam for 6-8 minutes until just cooked through.
2. Meanwhile, mix together the lime juice, the remaining fish sauce, the soy sauce and oil, and add the remaining chilli. Toss into the watercress with the cucumber, and serve topped with the fish.