Clean and wash the flounder under running water.
Cut the radish and onion into bite-sized pieces.
Place the radish at the bottom of a pot, then layer the flounder and onion on top.
Make a seasoning sauce by mixing soy sauce, gochujang (Korean chili paste), gochugaru (Korean chili flakes), sugar, garlic, ginger, spicy chili pepper, and water.
Pour the seasoning sauce over the ingredients.
Bring to a boil over medium heat; once it boils, simmer on low heat for 20–30 minutes.
Occasionally spoon the sauce over the fish while simmering.