Rinse the mackerel slices and pat them completely dry. Sauté chopped shallots and garlic in 1 tablespoon pork fat until fragrant.

Once the shallots and garlic are fragrant, turn off the heat and transfer them to a clay pot. Add the following seasonings: 2 tablespoons sugar, 1 tablespoon oyster sauce, 1 tablespoon seasoning powder, 1 tablespoon fish sauce, sliced chili, 1 teaspoon black pepper, and 3 tablespoons water. Turn the heat back on and stir until the mixture is well combined. Add the mackerel slices, quickly flip them once to coat both sides with the seasoning, then leave them undisturbed. Pour in enough boiling water to cover half the thickness of the fish. Simmer over low heat.

Continue simmering over low heat until the fish is tender and the sauce thickens. Add chopped green onions, then transfer to a serving plate.