Thinly slice the cucumber, preferably with a mandoline. Thinly slice the onion with a sharp knife.
Toast the sesame seeds in a small frying pan, over medium heat. Stir occasionally, until the seeds just begin to brown. Be careful not to burn them.
In a large bowl, gently toss everything together until evenly coated. Place the salad in a container with a tight fitting lid and refrigerate for at least an hour before serving. Gently toss again before serving.