In a small bowl, mix the cornstarch and water until smooth and set aside.
Heat olive oil in a large skillet or wok over medium-high heat.
Add the chicken pieces to the skillet and cook for 5-6 minutes, stirring occasionally, until the chicken is cooked through and lightly browned. Remove the chicken from the skillet and set aside.
In the same skillet, add the minced garlic, grated ginger, and sliced onion. Sauté for 2 minutes until fragrant and onions are translucent.
Add the sliced red bell pepper and zucchini to the skillet. Stir-fry for 4-5 minutes until the vegetables are tender-crisp.
Return the cooked chicken to the skillet with the vegetables.
Add the soy sauce, oyster sauce, black pepper, red chili flakes, and chicken broth. Stir well to combine.
Pour in the cornstarch slurry and stir continuously for 1-2 minutes until the sauce thickens and coats the chicken and vegetables evenly.
Remove from heat and sprinkle with chopped green onions.
Serve immediately.