Slice the beef against the grain into thin strips and marinate it with soy sauce, cornstarch, and sugar for about 10 minutes.

Whisk together oyster sauce, soy sauce, water or broth, and sesame oil with cornstarch to form a smooth sauce.

Heat oil in a large wok over medium-high heat and sauté minced garlic (and ginger if using) for 30 seconds until fragrant.

Add the marinated beef to the wok and stir-fry for 2-3 minutes until browned.

Add your choice of vegetables and the prepared sauce; stir everything together well and cook for an additional 2-4 minutes.

Combine everything in the wok and give it a final toss to coat all ingredients with the sauce.

Transfer to a serving dish and garnish with green onions or cilantro if desired, serve hot.