Slice the persian cucumbers and combine with the sliced onion, set aside.
In a bowl, whisk together the gochujang, gochugaru, water, salt, garlic, sugar, sesame oil, and sesame seeds. Add cucumbers and onions and toss to coat well. Let sit for 5-10 minutes before serving.
Notes
Persian cucumbers are long and narrow, having thin skin and very few seeds. You might also see English or "hot house" cucumbers at the store. Avoid regular cucumbers if you can.