Slice cucumbers thinly using a sharp knife or mandoline (about 1/8-inch thick).
Place sliced cucumbers in a colander and sprinkle with salt. Toss to coat evenly and let sit for 10 minutes to draw out excess moisture.
While cucumbers rest, make the dressing: In a small bowl, whisk together rice vinegar, soy sauce, sesame oil, honey, minced garlic, and red pepper flakes until well combined.
Rinse salted cucumbers briefly under cold water to remove excess salt, then pat dry with paper towels or a clean kitchen towel.
Transfer cucumbers to a large mixing bowl. Pour dressing over cucumbers and toss gently until all slices are coated.
Let the salad marinate for 10 minutes at room temperature, tossing once halfway through.
Just before serving, sprinkle with toasted sesame seeds and sliced green onions.
Toss one final time and serve immediately.