Heat olive oil in a skillet and sauté the onion for 3–4 minutes.
Add garlic and cook until fragrant, about 30 seconds.
Stir in tomato paste and cook 1 minute.
Add chopped tomatoes and simmer for 8–10 minutes.
Season with chili flakes, salt, and pepper.
Gently add canned mackerel and simmer 3–4 minutes.
Finish with vinegar or lemon juice.
Garnish with fresh parsley if using, and serve hot.