Step 1: In a medium bowl, combine the thinly sliced beef with 1 tbsp soy sauce, 1 tsp cornstarch, and 1/2 tsp sesame oil. Mix well and let marinate for at least 10 minutes while you prepare other ingredients.
Step 2: In a small bowl, whisk together the remaining 2 tbsp soy sauce, oyster sauce (if using), rice vinegar, brown sugar, 1 tsp sesame oil, minced garlic, and grated ginger. In a separate small bowl, whisk 1 tbsp cornstarch with 2 tbsp water to create a slurry. Set both aside.
Step 3: Heat 1 tbsp vegetable oil in a large wok or skillet over high heat. Add the marinated beef in a single layer and cook for 1-2 minutes per side until browned and just cooked through. Do not overcrowd; cook in batches if necessary. Remove beef from the pan and set aside.
Step 4: Add the remaining 1 tbsp vegetable oil to the hot wok. Add the sliced onions and stir-fry for 3-5 minutes, until softened and slightly caramelized.
Step 5: Return the cooked beef to the wok with the onions. Give the prepared sauce a quick stir and pour it over the beef and onions. Bring to a simmer.
Step 6: Stir the cornstarch slurry once more and slowly pour it into the simmering sauce while stirring continuously. Cook for 1-2 minutes, or until the sauce has thickened to your desired consistency. [IMAGE_PLACEHOLDER_0]
Step 7: Serve hot over steamed rice, garnished with chopped green onions if desired. Enjoy your homemade Chinese takeout!