In a bowl, combine beef slices with beef marinade ingredients. Mix well and marinate for 10 minutes.
Mix oyster sauce, cornflour/starch and cold water in a bowl. Set aside.
In a large non-stick pan, saute onion slices with 2 tablespoon of oil. Cook on medium-high heat until onions have turned lightly golden.
Add minced or sliced ginger, saute for about 10 seconds or until ginger becomes aromatic.
Add marinated beef slices and saute on high heat until lightly browned/caramelised and beef not cooked through.
Add kale leaves, continue to stir fry on high for about 1 minute or until kale has softened and significantly reduced in size.
Give the cornflour/starch mixture prepared in step #2 above a mix with a spoon (cornflour/starch would have settled at the bottom of the bowl). Pour mixture into the pan. Reduce heat down to medium, cook for 1 minute, mixing constantly.
Give the sauce a taste and season with salt and pepper to your liking. I like to go heavy on the pepper here for a little added heat.
Serve with rice.