Cucumber-Mint Salad

I got this recipe from a “how to cook Indonesian” cookbook years ago, and re-made it from memory the other day, to go with the sate-style chicken below. The freshness complements the chicken’s heavy flavors.
(To be honest, I never had a real Indonesian serve me a salad like this. They would usually serve diced cucumber, shallots and bird's-eye peppers floating in 20%-acetic-acid vinegar with no sugar added.)

Ingredients
4 cucumbers
¼ red onion, minced (optional)
¼ cup fresh mint leaves
¼ cup white vinegar (5% acetic acid)
3 T brown sugar or palm sugar

Method
For each cucumber, peel, cut in half crosswise, then lengthwise. Stand the cucumber quarters on end and use a spoon to scoop out the seeds. Discard seeds; slice the cucumbers thinly. You should get little crescent-shaped pieces. Place in a large bowl.
Mince the onion and add to the cucumber.
Wash the mint leaves, pat dry, and chop. Mix with the cucumber. (Shown)
Measure the vinegar into a measuring cup. Add the 3 T brown sugar and stir until dissolved. Pour over the cucumber and mint; stir. Cover and store in the fridge, stirring periodically so that all the cucumber gets a chance to marinade.