Place salmon in water to cover; soak 2 to 3 hours, changing the water several times. Drain Salmon. Remove skin, bones and white strings. Using spoon or fingers, shred salmon into small pieces; place with tomato and onion in bowl. Chill. Cover with thin layer of ice about 1 hour before serving. Add green onion just before serving. If salmon is unsalted, rub with rock salt and let stand overnight; rinse well and soak in water for 1 hour or longer, changing 2 or 3 times.
(You can use canned salmon, and get away with it on the mainland.)