Slice the mango to remove the stone. Cut the mango meat into small squares without piercing the skin. Scoop out with a spoon directly into a bowl.
Peel the cucumber, scoop out the seeds, then cut into cubes about the same size as the mango. Do the same with the tomatoes. Why remove the seeds? To prevent the salsa from turning watery.
Peel and roughly chop the onion.
Add the cucumber, tomatoes, onion, green onion and cilantro to the mango in the bowl. Pour in the rice wine and fish sauce. Sprinkle the sugar. Toss gently.