Begin by preparing your ingredients. Slice the beef into thin strips (about ¼ inch thick) and the onions into thin rounds. Mince the garlic.

In a small bowl, mix together the soy sauce, oyster sauce (if using), sugar, and cornstarch. Allow the beef to marinate in this mixture for about 10 minutes while you prep the rest of the ingredients.

Heat a large skillet or wok over medium-high heat and add the vegetable oil. Wait until the oil shimmers, which should take about 2 minutes.

Add the marinated beef to the hot skillet in a single layer and cook for 2-3 minutes without stirring.

Stir the beef, cooking for an additional 2-3 minutes until it reaches your desired doneness. Remove the beef from the skillet and set it aside on a plate.

In the same skillet, add the sliced onions and cook for about 5-6 minutes, stirring occasionally until they are soft and golden brown.

Add the minced garlic to the onions and sauté for an additional 30 seconds until fragrant.

Return the cooked beef to the skillet, tossing everything together for about 1-2 minutes to reheat. Adjust seasoning with salt and pepper to taste.

Serve hot, garnished with chopped chives or coriander.