Mix the vegetables: Place the cucumber slices and chopped green onion in a medium or large mixing bowl and set aside.
Make the dressing: In a small bowl, mix vinegar, soy sauce and garlic. Stir in sugar and salt and keep stirring until the sugar has dissolved.
Combine the dressing with the veggies: Pour the dressing over the cucumber slices and green onions and toss.
Add smokiness and serve: Sprinkle the Korean chili pepper flakes and sesame seeds and toss until the cucumber slices are evenly coated. Serve.
