1Put rice and water in a medium saucepan. Bring to a simmer over medium heat; cover and cook until the water is absorbed, about 20 minutes.
2Stir in lemon zest and juice.
3Meanwhile, bring broth to a simmer in another medium saucepan over medium-high heat. Reduce the heat so the broth remains steaming, but not simmering.
4Add fish and cook until just tender, about 5 minutes. Remove and break into bite-size chunks.
5Top between bowls with equal portions of the fish, watercress, carrot, mint and scallions. Ladle 1 cup of the warm broth into each bowl and serve.