1- Marinate the Chicken: In a bowl, mix together the chicken pieces with the soy sauce, oyster sauce, cornstarch, and sesame oil. Let it marinate for at least 15-20 minutes.

2- Prepare the Sauce: In a small bowl, whisk together chicken broth, soy sauce, oyster sauce, cornstarch, sesame oil, and sugar (if using). Set aside.

3- Blanch the Broccoli: Bring a pot of water to a boil. Add the broccoli florets and cook for 2-3 minutes or until slightly tender. Drain and set aside.

4- Cook the Chicken: Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the marinated chicken and stir-fry until it is no longer pink, about 4-5 minutes. Remove chicken from the pan and set aside.

5- Stir-fry the Vegetables: In the same pan, add a bit more oil if needed. Add the sliced onion and minced garlic, and stir-fry for about 2 minutes or until the onions become translucent.

6- Add Broccoli and Peas: Add the blanched broccoli and green peas to the pan. Stir-fry for another 2-3 minutes.

7- Combine and Cook: Add the cooked chicken back into the pan. Pour the sauce over the chicken and vegetables. Stir well to combine all the ingredients.

8- Cook for an additional 2-3 minutes or until the sauce thickens.

9- Taste and Adjust: Taste the dish and adjust the seasoning with salt or additional sauce, if needed.

10- Serve: Serve hot with steamed rice or noodles.