Heat the oil in a wok or skillet over medium heat. Stir-fry the garlic, ginger and mushrooms for 2 to 3 minutes, until fragrant. Add the hoisin sauce and stir-fry for a few seconds, then add the chicken stock and bring the mixture to a boil. Stir in the fish pieces and simmer uncovered, seasoning with the fish sauce, sugar and black pepper, until cooked, 3 to 5 mi