Slice ends off of your cucumber then thinly slice them into 2-3 mm disks (I recommend carefully using a mandolin for even cuts).
Add the cucumbers to a bowl, then mix together with salt and let them rest for 10 minutes. Drain out the excess water and taste for seasoning. If too salty for your tastes, rinse and pat them dry with a paper towel before continuing to step 3.
In the same bowl with the drained and dried cucumbers, combine with soy sauce, gochugaru, sugar, sesame seeds, onion, scallion, grated garlic, and sesame oil. Mix the cucumbers and seasonings until well combined and the cucumbers turn a deep red color.
Marinate the cucumbers for 10 minutes in the dressing before serving and enjoy!
Video
Notes
I recommend you use a seedless variety, such as English or Persian. Seedless cucumbers will help keep the sliced cucumbers crisp for longer. If you can't find a seedless cucumber, cut your cucumber lengthwise and before you begin slicing, remove the seeds in the middle with a spoon. (It doesn't have to be perfect, just scrap out what you can.) I like to cut them into 1/8" thin discs (2-3mm) because they're thin enough to soak up the dressing, while being thick enough to stay crunchy. However, you can cut them to whatever thickness or even shape you prefer - there is no wrong way! You can also cut them into half moons, thicker round slices, or 2" thin diagonal slices.Remove excess moisture from the cucumbers so they stay crisp - otherwise your Korean Cucumber Salad will turn soggy and watery! Salting for 10 minutes and draining the cucumbers will make a huge difference in the end result! Make sure to taste the cucumbers after draining to see if they're salty - if you think they are, rinse them with water and pat dry.Store Korean Cucumber Salad (Oi Muchim) in an airtight container in the refrigerator for 2-3 days. The cucumbers will lose water as they sit, which can lead to the cucumbers turning soft and diluting the flavors of the seasoning. I like to eat mine within the first 1-2 days, as I prefer my cucumbers on the crunchier side.