Combine all ingredients of the marinade in a large bowl. Whisk well until thoroughly combined. Transfer 1 ½ cups of the marinade into a saucepan. Place the chicken in the original bowl and toss well. Leave to marinate for at least one hour in the refrigerator.
Add cornstarch to the cold marinade in the saucepan. Whisk well until incorporated. Bring to a boil and simmer for 10 minutes, until thickened.
In a wok or skillet, heat peanut oil. Add onion and chiles. Stir fry until onions start to change color.
Strain chicken and discard the marinade. Add chicken pieces to skillet and stir fry until cooked through, about 5 minutes.
Add orange sauce and stir fry until chicken is well coated with the sauce. Simmer for 5 minutes.
Add sesame seeds and cook for 2 more minutes. Add salt if necessary.
To serve, garnish with more sesame seeds.