Soak the well-cleaned mackerel pieces in rice starch water for 10-15 minutes. Drain and set aside.
Meanwhile make the seasoning paste. Combine chili flakes, soy sauce, tuna sauce, garlic, ginger, sugar, and pepper in a small mixing bowl.
In a shallow braising pot, scatter the onion on the bottom of pan, and place radish slices evenly. Spread half the amount of chili seasoning sauce on top. Place mackerel pieces on top of the onions and radish, then spread the remaining sauce on the mackerel.
Put 1/2 cup of water in the chili seasoning sauce mixing bowl and swirl around to collect all the residue inside the bowl, then pour it into the pot.
Cover and bring it to boil. Reduce the heat to low and simmer for 15-20 minutes until radish is softened. Sprinkle with chopped green onion. Serve warm with rice.
Notes
To make rice water:
Rinse your rice with water briefly. Pour out the water and discard.
Swirl the rice around rapidly with your fingers for 15 seconds to remove starch from the rice grains.
Pour about 3 cups of water to the rice and rinse.
Collect the milky rice starch water in a bowl and add the mackerel pieces.
Continue to rinse your rice 2-3 more times. Cook your rice by your usual rice cooking method.