Heat the sunflower oil in a large wok or frying pan, add the red onion and pepper and stir fry for 1-2 minutes.
Add the garlic, chilli and chicken to the onion mixture and cook over a very high heat for about 4 - 6 minutes, or until chicken is sealed and tender.
Add the spring onions to the wok and cook for another minute. Place the cornflour in a small bowl and add the black bean sauce, sherry and water. Whisk until well combined. Add the sauce mixture to the wok and allow to thicken slightly.