In a bowl, combine the sliced top round steak, rice vinegar, soy sauce, brown sugar, minced garlic, and grated ginger. Mix well and let it marinate for at least 15 minutes.
In a separate bowl, whisk together the beef broth, cornstarch slurry, and additional soy sauce. Set aside.
Heat 2 tablespoons of peanut or vegetable oil over medium-high heat in a large skillet or wok until hot.
Add the marinated beef in a single layer to the skillet. Sear for 2-3 minutes until browned. Cook in batches if necessary. Remove beef from pan and set aside.
In the same skillet, add more oil if needed. Sauté the sliced onion, green bell pepper, and red bell pepper for 4-5 minutes until tender but crisp. Add minced garlic and sauté for 1 more minute.
Return the cooked beef to the skillet with vegetables. Pour the prepared sauce over and stir well. Simmer for 2-3 minutes until sauce thickens and coats the beef and vegetables.
Season with salt and freshly ground black pepper. Stir in sliced green onions and sprinkle sesame seeds on top.
Serve the Pepper Steak over hot cooked rice if desired. Enjoy!