Cook the lightly salted mackerel in a pan with a drizzle of olive oil for 2 minutes before removing them.
In the same pan, blanch the onion. Once cooked, deglaze the onion with the vinegar.
Add the garlic, chili, thyme, and salt, then reduce by half.
Pour over the mackerel, cover, and let infuse for 1 hour.
Serve hot or cold.