1. Mix the soy sauce and sugar together, stirring until the sugar has dissolved.

2. Cut chicken breasts into thin strips.

3. Place chicken into a shallow glass dish and spoon the soy sauce mixture over it, turning to coat completely.

4. Marinate in the refrigerator for 20 minutes.

5. Heat oil in a preheated wok.

6. Add chicken and stir fry for 2-3 minutes, until golden brown.

7. Add onions and garlic and cook for 2 minutes.

8. Add beansprouts and cook for 4-5 more minutes.

9. Add sesame oil.

10. Blend the cornstarch and water to form a smooth paste.

11. Pour the stock into the wok, along with the cornflour paste, and bring to a boil, stirring constantly until the sauce is thickened and clear.