Combine the soy sauce, ground coriander, mandarin zest, sugar and a pinch of cayenne pepper in a medium bowl. Add the lamb, tossing well to coat.
Heat 1 teaspoon of the oil in a large nonstick frying pan over moderately high heat. Add the lamb and stir-fry for 2 minutes or until lightly browned. Transfer the lamb to a plate with a slotted spoon. Reduce the heat to moderate, add the remaining oil, the capsicum, garlic and spring onions, and stir-fry for 4 minutes or until the capsicum is crisp-tender.
Whisk the stock into the cornflour in a small bowl. Add to the pan and bring to the boil. Return the lamb to the pan, add the mandarins and cook for 1 minute or until the sauce is slightly thickened. Garnish with the coriander leaves and spring onion spirals.