Wash the cucumbers thoroughly and pat them dry.
Slice into thick coins, about ½-inch wide.
Put the cucumber slices in a bowl and lightly sprinkle them with salt. Toss gently and let sit for 20–30 minutes to remove excess water.
While the cucumbers are resting, prepare the dressing by combining soy sauce, rice vinegar, sugar, sesame oil, minced garlic, and chili oil in a small bowl. Stir until the sugar dissolves completely.
After salting, rinse the cucumber slices under cold water to remove extra salt and pat them dry with a paper towel.
Mix the cucumber slices with the dressing until they are evenly coated. Cover the bowl and chill it in the refrigerator for at least one hour before serving. For best flavor, allow to marinate overnight.
Transfer to a serving plate and garnish with a drizzle of chili oil or sesame seeds if desired.
