In a large skillet or wok, heat the vegetable oil over medium-high heat.
Add the sliced onion and cook until translucent, about 2-3 minutes.
Stir in the minced garlic and ginger, cooking for an additional 1 minute.
Add the ground beef to the skillet, breaking it apart with a wooden spoon. Cook until browned, about 5-7 minutes.
While the beef is cooking, prepare the broccoli. If using fresh, steam or blanch the florets until bright green and tender-crisp, about 3 minutes. If using frozen, thaw and drain.
In a small bowl, mix the soy sauce, oyster sauce (if using), cornstarch, and water to create a sauce.
Once the beef is cooked, add the broccoli to the skillet and pour the sauce over the mixture. Stir well to combine.
Cook for an additional 3-4 minutes, allowing the sauce to thicken and the broccoli to heat through.
Season with salt and pepper to taste.
Serve warm over cooked rice.