Clean and cut the mackerel into pieces.
Slice the radish into flat pieces.
Layer the radish at the bottom of a pot and place the mackerel on top.
Make the sauce by mixing soy sauce, red pepper flakes, minced garlic, ginger, sugar, spicy green chili, and water.
Pour the sauce over the mackerel.
Bring to a boil over medium heat, then reduce to low and simmer until the sauce is reduced and the fish is cooked through.