Place 1 bunch scallions, cut into 4"-long pieces, very thinly sliced lengthwise, in a medium bowl of ice water and soak at least 10 minutes and up to 1 hour. Drain and pat dry with paper towels.
Transfer scallions to a clean medium bowl. Add 2 tsp. gochugaru (coarse Korean red pepper powder), 2 tsp. toasted sesame oil, and 2 tsp. toasted sesame seeds and toss gently to combine.
Just before serving, drizzle in 2 tsp. unseasoned rice vinegar and season with kosher salt; toss to combine. Do ahead: Scallion salad (without vinegar and salt) can be made 1 hour ahead. Cover and chill.
