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 sugar snap 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.
A mortar and pestle is ideal for crushing the peppercorns, but if you don't have one, put them in the corner of a strong plastic bag and press down with the end of a rolling pin or the back of a wooden spoon. Alternatively, some spice manufacturers sell mixed peppercorns in a pepper grinder, which makes the job very easy — look in the spice aisle of your local supermarket.
*If you prefer, you can use strips of chicken breast fillet instead of the fish. Cook until lightly browned. *You can be as adventurous as you like with the choice of vegetables. Try broccolini or halved beans instead of the sugar snap peas, or broad beans instead of the frozen peas.