Marinate the flank steak with the light soy sauce, cooking oil and corn starch and set aside for 15-30 minutes.
In a bowl, add all the sauce ingredients and mix thoroughly.
Heat 1 tablespoon of cooking oil on a wok or large pan over medium-high heat.
Add the bell peppers and onions and stir-fry until tender, approximately 5 minutes. Set aside on a plate.
Heat the remaining tablespoon of cooking oil on the same wok or pan and add the sliced ginger. Allow ginger to cook for about 30 seconds.
Add the marinated steak. Cook until steak is slightly pink, about 3-4 minutes.
Add the peppers and onions back into the pan with the steak.
Give the sauce mixture a quick whisk to loosen up the corn starch, then pour it into the pan.
Mix the sauce with the beef, onions and peppers, until the sauce thickens, about 2 minutes.
Top with a dash of black pepper (optional), mix then serve.