Clean the mackerel and cut into pieces. Marinate with garlic, shallots, 1 teaspoon sugar, 2 tablespoons fish sauce, 1 teaspoon seasoning powder, and black pepper. Let the fish marinate for 15 minutes. Slice the pineapple thinly.

Heat oil in a pan and sauté garlic and shallots until fragrant. Add 2 tablespoons sugar to make caramel. When the sugar turns amber, carefully add 1/2 cup boiling water (about 120 ml) over low heat to avoid splattering. Let the sauce simmer, then add the fish and pineapple. Braise over medium heat for about 30 minutes, turning the fish occasionally. Once the fish is well-cooked and the sauce thickens, turn off the heat.