Thinly slice the cucumber into 1/8-inch rounds and place in a clean jar or bowl. Finely chop the scallions and mince the garlic cloves, keeping them separate. Measure out the soy sauce, fish sauce, sesame oil, sugar, MSG, rice vinegar, sesame seeds, and chili oil. Having everything prepped and measured ensures the flavors will distribute evenly and the salad comes together quickly.
Add the scallions, minced garlic, soy sauce, fish sauce, sesame oil, sugar, MSG, rice vinegar, and sesame seeds directly to the jar with the cucumber. I like to add the garlic and fish sauce first—they're potent ingredients that coat the cucumber quickly and develop flavor. Close the jar tightly and shake vigorously for 30 seconds to distribute the dressing evenly and begin marinating the cucumber.
Add the chili oil or red pepper flakes and shake once more to incorporate. Pour the salad into a serving bowl or plate, making sure to drizzle all the dressing over the cucumber. I find that letting it sit for just 2-3 minutes before serving allows the cucumber to soften slightly while staying crisp, and the flavors meld beautifully.