Heat a wok or large skillet over medium-high heat and drizzle with oil.
Add ground beef and cook until browned, breaking it apart with a spatula. If using sliced beef, sear until just cooked through. Remove from pan and set aside.
In the same pan, sauté onion, garlic, and ginger until fragrant.
Add bell pepper, peas, and carrots, and cook until vegetables soften slightly.
Stir in the cooked rice, breaking up clumps. Let it toast for a minute or two.
In a small bowl, whisk soy sauce, hoisin sauce, brown sugar, oyster sauce, and sesame oil. Pour over the rice.
Return the beef to the pan and toss everything together. Add cornstarch slurry to thicken the sauce, coating the rice evenly.
Cook for another 2–3 minutes until everything is well combined.
Garnish with green onions and sesame seeds before serving.