Wash the cucumber and slice it thinly. Optionally, sprinkle a pinch of salt over the slices, toss, and let sit for 5 minutes to draw out excess water. Drain well.
In a bowl, combine the minced garlic, soy sauce, toasted sesame oil, dried chili flakes, fish sauce, and sugar. Stir until the sugar dissolves.
Add the cucumber slices and green onions to the dressing. Toss gently until evenly coated.
Sprinkle the toasted sesame seeds on top. Serve immediately for a crisp texture or refrigerate for 30 minutes to enhance the flavors.