Combine the vegetables. Put the cucumber, red onion, bell pepper (if using), and tomatoes in a large bowl.
9 cups diced cucumbers, 2 cups red onion, 1 med red bell pepper, 3 cups cherry or grape tomatoes
Dress it. Add the oil, rice vinegar (or mirin), soy sauce, and chili crunch. Toss well to coat.
1/4 cup neutral oil, 1/2 cup rice vinegar or mirin, 1/4 cup soy sauce, 1 tbsp chili crisp or chili crunch
Wait, if you can. Cover and refrigerate. It's good right away, better after a few hours, and best the next day.
Re-toss and taste before serving; wake it up with more chili crunch or a splash more vinegar if it needs it.