Step 1: Prepare the Beef. In a bowl, toss the thinly sliced beef with 1 tablespoon cornstarch. Set aside (this helps the beef brown and the sauce thicken).
Step 2: Make the Sauce. In a separate bowl, whisk together ¼ cup soy sauce, 3 tablespoons brown sugar, 2 tablespoons oyster sauce, 1 tablespoon hoisin sauce, 1 tablespoon rice vinegar, 1 tablespoon cornstarch, ½ cup beef broth, ½ teaspoon black pepper, and ½ teaspoon red pepper flakes (if using). Set aside.
Step 3: Sear the Beef. Heat 1 tablespoon vegetable oil in a large skillet or wok over medium-high heat. Add half the beef and cook until browned, about 2-3 minutes per side. Remove cooked beef from the skillet and set aside. Repeat with the remaining beef, adding a little more oil if needed.
Step 4: Stir Fry the Vegetables. Add the remaining 1 tablespoon vegetable oil to the skillet. Add the minced garlic, grated ginger (if using), and sliced bell peppers and onion (if using). Stir fry for 3-4 minutes, until the vegetables are crisp-tender.
Step 5: Combine. Return all the cooked beef to the skillet with the vegetables. Pour in the prepared stir fry sauce. Stir well to coat everything evenly and let it cook for 2-3 minutes, stirring occasionally, until the sauce thickens and becomes glossy.
Step 6: Serve. Spoon the hot beef and vegetable stir fry over steamed white rice. Garnish with sesame seeds or chopped green onions if desired, and serve immediately.