In a large skillet over medium heat, add the sesame oil. Once hot, add the diced onion and minced garlic. Sauté for about 2-3 minutes until the onion is translucent.
Add the ground beef to the skillet, breaking it apart with a spatula. Cook until browned and cooked through, about 5-7 minutes. Drain any excess fat if necessary.
Stir in the diced bell pepper, soy sauce, oyster sauce (if using), ground black pepper, and salt. Cook for an additional 3-4 minutes until the bell pepper is tender.
Add the cooked rice to the skillet, mixing well to combine all ingredients. Cook for another 2-3 minutes, allowing the rice to heat through.
Serve the beef pepper rice in bowls, garnished with chopped green onions and sesame seeds.