Clean the red snapper and cut it into bite-sized pieces.
Slice radish, onion, and green onion into suitable sizes.
Lay the radish in the pot and place the red snapper on top.
Mix soy sauce, mirin, sugar, and water to make the seasoning sauce.
Add the seasoning sauce and vegetables to the pot.
Bring to a boil over medium heat, then reduce to low heat to simmer.
When the sauce thickens, top with green onion to finish.