Heat vegetable oil in a large skillet or wok over medium-high heat. Add the thinly sliced beef and cook for 3-4 minutes, stirring frequently, until browned and cooked through. Remove the beef from the skillet and set it aside.
In the same skillet, add the onion and bell peppers. Cook for 4-5 minutes until the vegetables are tender but still crisp. Add the minced garlic and grated ginger, and cook for another 1 minute until fragrant.
In a small bowl, whisk together the soy sauce, oyster sauce, hoisin sauce, rice vinegar, sesame oil, brown sugar, and a pinch of salt and pepper.
Return the cooked beef to the skillet with the vegetables. Pour the sauce over the beef and vegetables, tossing everything together until well coated and heated through, about 2 minutes.
Serve the beef and pepper mixture over the cooked rice. Garnish with chopped green onions and sesame seeds, if desired.