In a large mixing bowl, combine the thinly sliced flank steak with 2 tbsp soy sauce, 1 tbsp cornstarch, 1 tbsp sesame oil, 1 tsp grated fresh ginger, 1 minced garlic clove, and 1/2 tsp black pepper. Mix well to ensure all beef strips are coated. Cover and refrigerate for at least 30 minutes, or up to 2 hours.

In a small bowl, whisk together the beef broth, 2 tbsp soy sauce, oyster sauce, rice vinegar, sugar, and 1 tsp cornstarch until smooth. Set aside.

Heat 1 tbsp vegetable oil in a large skillet or wok over medium-high heat. Add half of the marinated beef in a single layer (do not overcrowd the pan). Cook for 2-3 minutes per side, until browned and cooked through. Remove the cooked beef from the skillet and set aside. Repeat with the remaining beef, adding another 1/2 tbsp of oil if needed.

Add the remaining 1/2 tbsp vegetable oil to the same skillet. Add the sliced yellow onion, red bell pepper, and green bell pepper. Stir-fry for 3-5 minutes, until the vegetables are tender-crisp. Add the 2 minced garlic cloves and cook for another 30 seconds until fragrant.

Return the cooked beef to the skillet with the vegetables. Give the prepared sauce a quick whisk again (cornstarch tends to settle) and pour it over the beef and vegetables. Stir constantly for 1-2 minutes, allowing the sauce to thicken and coat everything evenly.

Divide the cooked white rice among four serving bowls. Spoon the beef and pepper mixture generously over the rice. Garnish with sesame seeds and sliced green onions, if desired. Serve immediately.