Clean and prepare the Spanish mackerel thoroughly.
Cut radish and onion into bite-sized pieces.
Lay the radish and onion at the bottom of a pot.
Place the prepared Spanish mackerel on top.
Mix gochujang (Korean red chili paste), soy sauce, gochugaru (Korean red chili flakes), minced garlic, sugar, mirin, and water to make the seasoning sauce.
Pour the sauce over the Spanish mackerel.
Simmer gently.
Slice Cheongyang chili pepper and green onion, add them on top, and simmer once more.