Step 1

Place onion and scallions in a medium bowl of cold water. Chill until scallions curl, at least 20 minutes.

Step 2

Meanwhile, whisk gochugaru, sugar, 1 tsp. salt, ½ tsp. pepper, and 1 Tbsp. water in a medium bowl. Let sit, whisking occasionally, until sugar is dissolved and sauce looks shiny, about 10 minutes.

Step 3

Drain onion and scallions and spin in a salad spinner or pat dry. Transfer to bowl with gochugaru sauce. Add vinegar and toss to coat; season with salt and pepper.

Step 4

DO AHEAD: Onion and scallions can be soaked 1 day ahead. Keep chilled.