Steps

1. Finely chop 1 onion.
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2. Heat 20g of butter in a frying pan and sauté the chopped onion until caramelized. (Key Tip!) Caramelizing the onions well brings out their sweetness, making the topping for the mackerel delicious. You can also use olive oil or a combination of olive oil and butter.
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3. Sprinkle salt all over 1 mackerel. (Key Tip!) Letting it sit for about 10 minutes after salting allows the salt to penetrate the fish.
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4. Roughly chop 100g of seedless ripe olives.
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5. Finely chop 10g of parsley, 10 sage leaves, 5g of thyme, and 5g of basil (2 sprigs) all together. (Key Tip!) If fresh herbs are not available, dried herbs can be used. Even without herbs, you can make a delicious dish with onions, black olives, mackerel, and acidity.
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6. Add the roughly chopped ripe olives and chopped herbs to the caramelized onions and mix lightly. Turn off the heat.
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7. Add a tiny pinch of salt to the mixed topping.
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8. Lightly grease a baking dish with olive oil and arrange the salted mackerel.
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9. Place the mixed topping of sautéed onions, olives, and herbs over the mackerel. Arrange them to cover the fish pieces closely.
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10. Bake in a preheated oven at 200°C (400°F) for about 20 minutes.
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11. Remove from the oven and generously squeeze 1/2 lemon over the hot mackerel while it's still piping hot.
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12. Drizzle a little white wine vinegar over the hot mackerel and it's ready to serve.
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