Prepare all the vegetables buy cut them into thin slices and cut the chicken breast into thin slices. Marinade the chicken slices with corn starch, sesame oil, sugar, baking soda and light soy sauce. Mix it well with hand and set aside for 10 to 15 minutes.
In a small mixing bowl add the oyster sauce, light soy, sesame oil, sugar, corn starch, white pepper and vinegar. Mix well until sugar and con starch dissolve and set aside.
Heat the large wok/pan into medium-high heat, drizzle 2 tablespoons of vegetable oil and stir fry the chicken slices 2-4 minutes until cooked through. Remove the chicken from pan and set aside.
Clean the remaining crust from the pan, drizzle 1 tablespoon of oil, add onion slices and dry chillies and stir fry for few seconds, follow with the garlic slices, mushroom, and bell pepper slices. Continue cook for another 1-2 minutes and place the chicken slices back in.
Pour the sauce mixture and toss well to combine the sauce and the ingredients evenly. Stir in the spring onions.
Turn off heat and transfer to serving plate. Garnish with toasted sesame seeds and extra chopped spring onions. Perfect to serve together with hot steamed rice or stir fry noodles.