Peel the cucumber and onion, cut into small cubes, sprinkle with a little salt and pepper and let stand for 15 minutes.
In the meantime, slit the chili pepper lengthways, core and finely dice. Mix together rice vinegar, rice wine, soy sauce and sugar. Beat in the sesame oil with a whisk. Mix the cucumber, chilli and sauce and let it steep for another 30 minutes.