Thinly slice the cucumbers into ⅛-inch rounds using a mandoline or sharp knife.
Place cucumber slices in a colander set over a bowl, sprinkle with salt, and toss to coat. Let drain for 10 minutes to remove excess moisture.
While cucumbers drain, whisk together minced garlic, soy sauce, rice vinegar, sesame oil, and sugar in a small bowl until sugar dissolves.
Rinse cucumbers briefly under cold water to remove excess salt, then pat dry with paper towels or a clean kitchen towel.
Combine cucumbers and dressing in a large mixing bowl, tossing gently until evenly coated.
Let salad marinate at room temperature for 15 minutes to allow flavors to develop.
Just before serving, sprinkle with toasted sesame seeds and green onions.
Toss once more and transfer to a serving bowl.
