In a small bowl, mix together soy sauce, oyster sauce, sugar, black pepper, and salt. Set aside.
In a separate bowl, dissolve cornstarch in water and set aside.
Heat rapeseed oil in a wok or large frying pan over medium-high heat. Add the beef strips and stir-fry until they are browned and cooked through. Remove the beef from the pan and set aside.
In the same pan, add the sliced onion, red pepper, green pepper, chopped garlic, and ginger. Stir-fry for 2-3 minutes until the vegetables are slightly softened.
Return the cooked beef to the pan and pour the sauce mixture over it. Stir-fry for an additional 1-2 minutes, ensuring the beef and vegetables are evenly coated with the sauce.
Give the cornstarch mixture a quick stir and then add it to the pan. Stir continuously for another minute or until the sauce thickens and coats the beef and vegetables.
Remove from heat and transfer to a serving dish. Garnish with fresh coriander leaves, if desired.
Serve hot with rice or noodles.