Wash the cucumber with salt, scrubbing the skin clean. Remove any spiky parts with the back of a knife. Slice the cucumber in half and cut it diagonally into thick pieces for better texture.

Slice half an onion to match the size of the cucumber pieces. Place the sliced cucumber and onion in a large bowl.

Add gochujang, red pepper flakes, sugar, soy sauce, and vinegar to the bowl. Mix well. For a milder taste, omit gochujang and increase red pepper flakes.