Prepare the mackerel by removing the scales and guts.
Slice the radish and onion into bite-sized pieces.
Layer the radish at the bottom of a pot, then place the cleaned mackerel and onion on top.
Mix soy sauce, red chili powder, minced garlic, ginger, and water to make the sauce.
Pour the sauce into the pot and simmer over medium heat.
When reduced, add green onions and spicy chili peppers, then simmer a bit more.