In a small pan, heat together the soy sauce, sugar and mirin for about 5 minutes until the sugar is dissolved and the mixture is just syrupy. Leave to cool.
Thread alternate pieces of chicken and leek onto 10-12 short skewers. Place skewers in a bowl or plastic container and pour over the soy sauce mixture. Cover and leave to marinate in the fridge for about 1 hour, turning half way through.
Heat the sunflower oil in a large nonstick frying pan and cook the skewers over a medium heat for about 5 minutes per side.
Once all the skewers are cooked through with no pink showing, remove from the pan and add the remaining sauce, cook for a few minutes until piping hot and slightly thickened. Spoon over the skewers and serve.
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