Put the leek in a large microwave dish with 4 tbsp of the stock. Cover the dish with cling film, pierce the film with a knife, then microwave on High for 5 mins.
Uncover the dish, then stir the rice and remaining stock into the leek. Re-cover with cling film, pierce and microwave on High for another 10 mins, stirring halfway through until the rice is very nearly cooked.
Gently stir in the fish chunks, cover the dish with cling film again, then pierce and cook for a further 5 mins until the fish flakes easily and the rice is tender. Stir in the parsley, lemon zest and juice. Leave to stand for 2 mins before serving.