Sauce: Combine oyster sauce, soy sauce, water, ground black pepper, sesame oil, and chili flakes and cornflour/cornstarch in a small bowl. Whisk together and then set aside. (note: you can omit the cornstarch/cornflour and it wont affect the taste, however the sauce will have a thinner consistency)
Prepare and cook Chicken: Chop chicken breast into bite-sized pieces. Heat 1 tablespoon of olive oil in a large wok and cook chicken for two minutes until golden brown and cooked through. Transfer the chicken to a plate and cover.
Cook and season Vegies: Add 1 tablespoon of olive oil into the wok, followed by sliced red bell pepper/red capsicum and broccoli florets and stir until just tender. Add sliced mushrooms and cook for a further minute or until they are cooked to your liking. Add minced garlic and stir for one to two minutes.
Add Chicken back into the Wok: With both chicken and veges in the wok, whisk the sauce first, and then pour over, stirring quickly to combine the sauce, chicken and veges. It will thicken quickly if using cornflour/cornstarch.
Taste Test: Taste the sauce and add salt if needed. Remove wok from heat as soon as the sauce has reached the consistency you want. (if it becomes too thick, add little more water to loosen).
Serve: divide between 4 bowls and serve with your favorite rice or noodles, and garnish with sesame seeds, extra chilli flakes and a slice of lemon.