Peel the cucumber and cut into half-inch cubes.
Cut the tomatoes into halves and squeeze out the seeds. Cut the deseeded tomatoes into half-inch cubes.
Thinly slice the finger chili.
Cut the scallions into half-inch lengths.
Peel and roughly chop the onion.
In a bowl, mix together the lemon juice, fish sauce and sugar.
Place all the cut vegetables in a bowl. Drizzle the dressing over them. Toss lightly but thoroughly.
Garnish the cucumber, chili and tomato salad with torn cilantro before serving.