Thoroughly clean the mackerel: remove the guts, head, and tail, then slice into smaller pieces.
In a large bowl, combine the water and salt, then add the mackerel slices. Let it sit for 1 hour to prepare the fish for pickling.
Rinse the mackerel with cold water, then layer it in a sterilized jar with thinly sliced red onion, garlic, dried chili peppers, and sliced jalapeño peppers.
In a saucepan, bring the vinegar, water, brown sugar, bay leaves, black peppercorns, mustard seeds, cumin seeds, and dill to a boil. Let it cool to room temperature.
Pour the cooled spicy brine over the mackerel, ensuring it completely covers the fish pieces.
Seal the jar and refrigerate for at least 24 hours to allow the flavors to meld.
Let it sit in the refrigerator for at least 48 hours before serving to allow the flavors to fully develop.
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