Add thinly sliced beef to a resealable bag with the marinade ingredients. Mix everything so the beef is well coated. Refrigerate for at least 20 minutes.

Heat a large skillet over medium-high heat and add a generous drizzle of vegetable oil. When the oil is hot, add the beef and cook for one minute on each side. Remove from the pan to a clean plate lined with paper towels. It works best to cook in batches to ensure you don't overcook the meat.

Add another drizzle of vegetable oil to the pan and cook the onions for 1-2 minutes. Add the Bok choy and continue cooking for 2-3 minutes until the vegetables are tender.

Return the beef to the pan with the vegetables and toss with oyster sauce, soy sauce and sugar. Cook just for a minute, stirring constantly, until the sauce has slightly thickened.
Serve over rice or stir-fry noodles and optionally garnish with sesame seeds and green onions.