Step 1: In a medium bowl, combine the sliced beef with 1 tablespoon of soy sauce and 1 tablespoon of cornstarch. Let it marinate for at least 15 minutes.
Step 2: In a small jar or bowl, whisk together the beef broth, oyster sauce, dark soy sauce, sesame oil, sugar, and the remaining teaspoon of cornstarch to create the stir-fry sauce.
Step 3: Heat 1 tablespoon of oil in a wok or large skillet over high heat. Add the beef in a single layer and sear until browned on both sides. Remove the beef and set aside.
Step 4: Add the remaining tablespoon of oil to the pan. Toss in the onion wedges and stir-fry for 2-3 minutes until they begin to soften but still have a crunch.
Step 5: Add the garlic and ginger to the onions and cook for another 30 seconds until fragrant.
Step 6: Return the beef to the pan. Pour the sauce over the mixture and toss continuously for 1-2 minutes until the sauce has thickened and coated everything evenly.
Step 7: Serve the chinese beef and onion stir fry immediately over hot rice.