In a small bowl, combine 1/4 cup of canned mackerel juice with garlic, ginger, Korean chili flakes, and soy sauce. Add the kimchi juice and combine well. Set aside.
Heat grape seed oil in a shallow pot over medium heat and fry kimchi for 5 minutes until they get somewhat soft. (Add 1 teaspoon sugar if your kimchi is too sour)
Add canned fish and onion to the pot and drizzle the chili garlic sauce over. Pour water to barely over everything. You might nee a little more.
Bring to boil and cover, reduce the heat to low and simmer for 15-20 minutes. Taste the stew and season more with soy sauce if you need.
Add the chilies, if using, and cook for 1-2 more minutes. Garnish with green onion. Serve hot with rice.