Slice cucumbers into thin rounds, about ⅛ to ¼ inch thick. Place in a colander, toss with the kosher salt, and let sit 10 to 15 minutes to draw out excess moisture. Pat dry gently with paper towels.
In a large bowl, whisk together the gochugaru, gochujang, soy sauce, sesame oil, rice vinegar, sugar, and garlic until the gochujang is fully incorporated.
Add the drained cucumbers, green onions, and fresh chili pepper if using. Toss well until every slice is evenly coated.
Taste and adjust - more gochugaru for heat, vinegar for tang, or sugar for balance.
Garnish with toasted sesame seeds. Serve immediately for best crunch, or refrigerate 10 to 15 minutes for more developed flavor.
