Cut the fish fillets into strips and set aside. Heat a wok or large frying pan over a high heat. Add the oil and, when it is very hot and slightly smoking, turn the heat down to medium and add the fish strips. Fry these gently for about 2 minutes or until they are brown on both sides, taking care not to break them up. Remove with a slotted spoon and drain on kitchen paper. Drain off most of the oil from the wok, leaving about 1½ tablespoons.

Reheat the wok, add the groundnut oil for the sauce, plus the black beans, garlic, ginger and onion, and stir-fry for 2 minutes. Then add the chillies, button mushrooms and spring onions and stir-fry for another 3 minutes over a high heat.

Now add the soy sauce, lemon juice, sugar, salt and pepper. Stir-fry for 10 seconds. Return the fish to the wok and gently finish cooking it in the sauce for about 1 minute. Give the mixture a good stir. Using a slotted spoon, arrange the fish on a warm serving platter, garnish with the spring onions and serve at once.