In a mixing bowl, combine the chicken pieces with soy sauce, oyster sauce, and cornstarch. Toss well to coat and let it marinate for at least 30 minutes.

Heat the vegetable oil in a large skillet or wok over medium-high heat. Once hot, add the marinated chicken in a single layer. Cook for about 5-7 minutes until the chicken is browned and cooked through. Remove the chicken from the pan and set aside.

In the same skillet, add the sliced onion, red bell pepper, and green bell pepper. Stir-fry for about 3-4 minutes until the vegetables are slightly tender.

Add the minced garlic, ginger, and sliced red chilies to the pan. Stir-fry for another 1-2 minutes until fragrant.

Return the cooked chicken to the skillet. Add sweet chili sauce, sesame oil, and sugar. Toss everything together and cook for an additional 2-3 minutes until heated through.

Season with salt and pepper to taste. Remove from heat and garnish with chopped spring onions and sesame seeds if desired.

Serve hot with steamed rice or noodles.