Step 1: In a medium bowl, combine the thinly sliced beef with 1 tablespoon of soy sauce, 1 tablespoon of cornstarch, and 1 teaspoon of vegetable oil. Mix well to coat the beef evenly. Let it marinate at room temperature for 15-20 minutes.
Step 2: In a small bowl, whisk together the remaining 2 tablespoons of soy sauce, 1 tablespoon of oyster sauce, 1 teaspoon of sugar, 1 teaspoon of sesame oil, 1 teaspoon of cornstarch, and 1/4 cup of water or beef broth until smooth. Set aside.
Step 3: Heat a large wok or heavy-bottomed skillet over high heat until it just begins to smoke. Add 1 tablespoon of vegetable oil.
Step 4: Add about half of the marinated beef to the hot pan in a single layer. Sear for 1-2 minutes per side until browned. Remove the cooked beef and set it aside. Repeat with the remaining beef, adding another teaspoon of oil if needed.
Step 5: Add the sliced onion to the pan (add more oil if necessary). Stir-fry for 3-4 minutes until the onions begin to soften and lightly caramelize.
Step 6: Add the minced garlic and grated ginger to the pan with the onions. Stir-fry for about 30 seconds until fragrant, being careful not to burn the garlic.
Step 7: Return all the cooked beef to the pan with the onions and aromatics. Give the prepared stir-fry sauce a quick whisk again and pour it over the ingredients.
Step 8: Stir continuously for 1-2 minutes as the sauce thickens and coats the beef and onions, becoming glossy. Ensure everything is well combined and heated through.
Step 9: Remove from heat immediately. Taste and adjust seasoning if needed. Serve hot over jasmine rice, garnished with green onions or sesame seeds if desired.