In a large skillet, heat the olive oil over medium heat. Add the diced onion and cook until translucent, about 3-4 minutes.

Add the ground beef to the skillet, breaking it apart with a spatula. Cook until browned and fully cooked, about 5-7 minutes. Drain any excess fat.

Stir in the minced garlic and cook for an additional 1 minute until fragrant.

Add the sliced red and green bell peppers to the skillet. Cook for about 5 minutes, or until the peppers are tender but still crisp.

Pour in the soy sauce, oyster sauce, and sesame oil. Stir to combine and let it simmer for 2-3 minutes. Season with salt and pepper to taste.

Serve the beef and pepper mixture over the cooked white rice. Garnish with chopped green onions and sesame seeds if desired.