Heat a wok or large frying pan over a high heat and add 1 tablespoon of the oil. Add the onion and stir-fry for 1 minute. Stir in the capsicums and snow peas and continue stir-frying for 3–5 minutes until the vegetables are tender but still crisp. Transfer the vegetables to a plate using a slotted spoon and keep them in a warm place.
Add the remaining oil to the wok and heat, then add the fish and stir-fry over a gentle heat for about 4 minutes, carefully turning the chunks so as not to break them up, until the fish is cooked through and flakes easily when tested with the point of a knife.
Add the lemon zest, lemon juice and crushed peppercorns to the wok. Return the stir-fried vegetables to the wok together with the peas and bean sprouts. Heat through, stirring, for 2–3 minutes. Scatter the basil over the top and serve at once.