Toss the sliced cucumbers and shallots with the salt in a medium bowl and let stand for 20 minutes, tossing every 10 minutes.
Rinse and drain the cucumbers and shallots and wipe out the bowl with a paper towel. Put a third of the mixture in cheesecloth and squeeze to remove all the moisture. (The cucumbers will be slightly bruised.) Return the squeezed mixture back to the bowl. Repeat in batches with the remaining cucumbers and shallots.
Add the gochugaru, sesame oil, vinegar, fish sauce and garlic and toss until the cucumbers are completely coated. Sprinkle with sesame seeds before serving.