Peel the cucumber and slice lengthwise. Scoop out the seeds and discard. Cut the cucumber into thin slices.
Combine the cucumber, onions, and salt in a small bowl. Mix, then let rest for approximately 5 minutes. Squeeze cucumbers to remove water, then transfer to dry bowl.
Add the tomato pieces and cilantro. Then stir in the yogurt and cumin. Turn gently with a spoon to combine. Cover and refrigerate for 1 hour before serving.