Soak the grated carrot in the rice wine vinegar to one side. Place the rice and the boiling water together in a saucepan and bring to the boil over a moderate heat. Cover with a tight-fitting lid and reduce to a very low heat. Simmer for 15 minutes until the water has been absorbed by the rice. Remove from the heat and keep covered to one side.
Combine the pineapple juice, soy sauce and grated ginger in a saucepan. Bring to a simmer over a moderate heat, then reduce the heat and simmer the salmon in it for 10 minutes. Remove from the heat and set to one side after 10 minutes.
Heat the sesame oil in a large wok over a high heat until hot. Stir-fry the spinach/chard and spring onion greens for 1 minute, then remove from the heat. Reheat the seaweed in the wok if using, at this point. Remove the lid from the rice and fluff up the grains with a fork. Spoon into serving bowls.
Remove the salmon from the poaching liquor and flake roughly using a fork. Spoon on top of the rice and top that with the seaweed if using. Spoon the spinach/chard and spring onions on top and garnish the dish with the sesame seeds and some of the grated carrot before serving.