1 large head cabbage,1/4 cup canola oil, 1 onion, diced, 2 teaspoons minced garlic, 4 roma (plum) tomatoes, diced, 1 cup water, 1 (6 ounce) can tomato paste, 1 (15 ounce) can mackerel in tomato sauce, 1 pinch red pepper flakes, or to taste, 1 pinch seasoned salt, or to taste
Place cabbage into a large pot and cover with salted water; bring to a boil. Reduce heat to medium-low and simmer until just tender, about 10 minutes. Drain.
Heat canola oil in a saucepan over medium heat. Cook and stir onion and garlic in hot oil until onion is translucent, 5 to 7 minutes.
Stir tomatoes into onion mixture, place cover on saucepan, and cook until tomatoes are soft, 10 to 15 minutes.
Mix water and tomato paste in a small bowl until smooth; pour into the saucepan. Stir the mixture, replace the cover, and cook another 5 minutes.
Add mackerel in tomato sauce; season with red pepper flakes and seasoned salt. Stir drained cabbage into the mixture.
Cover saucepan and simmer mixture until mackerel is hot, 10 to 15 minutes.