Clean and rinse the mackerel. Rinse the onion skins.
In a pot, combine 2 liters water, 8 tablespoons salt, 4 tablespoons sugar, onion skins, 3 teaspoons black tea (no additives), 7 bay leaves, and 10 allspice peas. Heat to medium-high until boiling, then reduce to medium and simmer for 5 minutes.
Strain the brine and let it cool to room temperature.
Place the mackerel in a container and pour the cooled brine over it. Cover and refrigerate.
Refrigerate for 4 days until the mackerel is fully cured and ready to serve.