Put 1/2 cup water and 1/2 teaspoon salt in a large pot and spread bean sprouts over. Close with a lid and bring the pot over med-high heat and cook for 6 minutes (You will need to lower the heat a little after about 3 minutes). Do not open the lid during the cooking time. When done, toss the bean sprouts in the liquid first, and then strain them out. Discard the liquid.
Place the sprouts in a large mixing bowl and add chili flakes, green onion, garlic powder, Korean say sauce for soup, sesame oil, and the sesame seeds, and fresh red chili (if using). Toss well to combine with your hand to incorporate the seasoning with the sprouts. Season with salt according to your taste.