Heat vegetable oil in a large skillet over medium-high heat.
Add minced garlic and ginger, sautéing until fragrant.
Add ground beef, breaking it apart with a spatula.
Cook until browned, about 5-7 minutes.
Season with salt and pepper.
Add sliced onions and cook for another 2-3 minutes.
Stir in broccoli florets and bell peppers.
Drizzle soy sauce and oyster sauce over the mixture.
Stir-fry the vegetables until they are tender-crisp, about 5 minutes.
Serve hot over rice or noodles.