How to Make It
In a large mortar, pound the garlic to a paste with the chiles, thickly sliced shallot and sugar. Add the cilantro and pound to a paste. Stir in the fish sauce, lime juice and water.
In a large skillet, cook the pork fat over moderately low heat until 1/2 cup of fat has rendered, about 20 minutes. Discard any solid bits of fat. Season the mackerel with salt and pepper. Heat the fat until shimmering. Add the fish, skin side down, and cook over moderately high heat until browned and crisp, about 4 minutes. Reduce the heat to moderate, turn the fillets and cook for 2 minutes longer. Transfer to plates. Add the thinly sliced shallots to the skillet and fry, stirring, until browned and crisp. Scatter the shallots over the fish. Spoon some of the cilantro sauce alongside and pass the rest of the sauce at the table.