Make the kelp stock by soaking a small piece of kelp (about 4” length) into a pot with l 1/2 cups of water - boil, then simmer 10-15 min. Set aside 1 cup of stock.
Cut the head and tail off off of the mackerel, and also remove the intestines. Rinse and pat dry with a paper towel. Cut into about 2" thick pieces (about 4-6 pieces per mackerel).
Prepare Sauce A - mix all ingredients together, then spread over the mackerel chunks, letting it marinate for 10-15 minutes.
Cut radish into about ½" thick slices, then quarter the slices. Also slice the onion into l/4” thick slices. Cut green pepper, red pepper and leeks diagonally. Set aside.
Put the onion and white radish slices at the bottom of a pot, then layer the mackerel pieces on top. Pour Sauce B mixture evenly to cover all the mackerel pieces.
Pour the 1 cup of saved kelp stock in the edge of the pot (so it doesn't disturb the Sauce B mixture). Bring it to boil and cook on the medium heat for 10-15 minutes. When half of the liquid is gone, add green peppers, red peppers, and leek pieces. Scoop the sauce from the bottom of the pot over the fish several times to coat the top. When most of the liquid is gone, remove from the heat. Serve warm.