In a large wok or frying pan over high heat, sizzle the oil. Toss in the chicken and stir-fry in 2 batches for 5 minutes until golden and fully cooked.
Combine the onion, capsicum, beans, and chilli in the pan. Stir-fry for 3 minutes until the onion is tender.
In a small bowl, mix soy sauce, tomato sauce, vinegar, and sugar. Add the chicken back to the wok with the sauce. Stir-fry for 2 minutes until the sauce boils and thickens slightly.
Portion the rice and distribute evenly among individual serving bowls for a delightful presentation.