Slice the cucumbers: Cut them into 1/4-inch thick coins or half-moons. Keep the pieces even so they salt and season uniformly.
Salt and rest: Place cucumbers in a bowl and toss with 1 teaspoon kosher salt. Let sit for 10 minutes.
This step pulls out excess moisture and keeps the salad crunchy.
Make the dressing: In a separate bowl, mix rice vinegar, gochugaru, sugar, soy sauce, garlic, and sesame oil. Taste and adjust—add more vinegar for brightness, or more gochugaru for heat.
Drain the cucumbers: Pour off the released liquid. Gently squeeze the cucumbers by hand to remove a bit more water.
Don’t crush them; a light press is enough.
Toss to coat: Add the cucumbers to the dressing. Toss until every slice is well coated and glistening.
Finish with aromatics: Stir in scallions and sesame seeds. If using fish sauce, add it now and toss again.
Rest briefly (optional): Let the salad sit for 5 minutes to meld flavors.
Serve chilled or at room temperature.