Cut hairtail fish, radish, onion, and green onion into bite-sized pieces.
Layer the radish on the bottom of a pot and place the boneless hairtail fish on top.
Add onion, green onion, and spicy green chili.
Make a seasoning sauce with gochujang, soy sauce, sugar, minced garlic, red pepper powder, and pepper.
Spread the seasoning sauce evenly over the ingredients.
Pour 200ml of water and cover with a lid to boil.
Simmer over medium heat for 15–20 minutes until the sauce thickens slightly.
When the sauce has reduced and the ingredients are cooked, it's ready to serve.