Step 1: Prepare the Beef

1. Slice the beef thinly against the grain. This ensures tenderness.
2. In a medium bowl, mix the sliced beef with 1 tablespoon of soy sauce and 1 teaspoon of cornstarch (if using). Let it marinate for about 15 minutes to enhance the flavor.

Step 2: Prepare the Sauce

1. In a small bowl, whisk together soy sauce, oyster sauce, brown sugar, rice vinegar, hoisin sauce, sesame oil, and cornstarch dissolved in water.
2. Set the sauce aside for later use.

Step 3: Cook the Beef

1. Heat 1 tablespoon of vegetable oil in a large skillet or wok over high heat.
2. Add the marinated beef slices in batches. Cook for about 2-3 minutes, turning frequently until browned but not overcooked.
3. Remove the beef from the skillet and set aside.

Step 4: Stir-Fry the Vegetables

1. In the same skillet, add another tablespoon of oil.
2. Add minced garlic, minced ginger, and sliced onion. Sauté for 1-2 minutes until fragrant.
3. Add the broccoli florets and red bell pepper slices (if using). Stir-fry for 4-5 minutes until the vegetables are crisp-tender.

Step 5: Combine and Sauce It Up

1. Return the cooked beef to the skillet.
2. Pour the prepared Soy Ginger Garlic Sauce over the beef and vegetables.
3. Stir well to combine, ensuring everything is coated with the sauce. Cook for another 2-3 minutes until the sauce thickens slightly.
4. If the sauce is too thin, add a cornstarch slurry (1 tablespoon cornstarch mixed with 1 tablespoon water) and stir until it reaches the desired consistency.

Step 6: Serve

1. Garnish with sliced green onions.
2. Serve hot over steamed rice or noodles for a complete meal.