Mix sauce ingredients: light soya sauce, oyster sauce, sugar, water and sesame oil.
Pat the beef slices dry with kitchen paper. Removing excess moisture helps the meat brown properly.
Heat a non-stick pan over medium-high heat for about 1 minute. Add the oil.
Spread the beef out in one layer. Leave the beef untouched for about 20–30 seconds so it can start to brown. I like my beef super caramelised so I flip the beef and cook the other side.
Now, for the aromatics. Add onions to the pan. Cook until the onions soften and then add in garlic.
When the edges of the onions start turning lightly golden, add the sauce ingredients and let cook until it starts to thicken.
Return the beef to the pan. Toss quickly until the beef is well-coated with the sauce.
Finish with a crack of black pepper. Toss through and serve hot with white rice.