In a mixing bowl, mix together hoisin sauce, tomato sauce, sherry, pepper, soy sauce, sugar and salt. Set aside for 20 minutes.
Rub fish filets with 1 tablespoon of oil. Heat a large frypan and add the remaining 2 tablespoons of oil. Fry fish for 2 minutes on both sides, drain on paper towels and set aside.
Discard oil from frypan and add 1 tablespoon of lard. Melt over medium heat and add soy sauce mixture, stirring well. Dissolve the cornflour in the water and pour into the frypan, stirring constantly. Cook until thickened.
Return fish to the frypan and cook again for 1 minute on each side.