Heat vegetable oil in a large skillet or wok over medium high heat.
Add minced garlic and ginger, sautéing for about 30 seconds until fragrant.
Add the ground beef, breaking it apart with a spatula. Cook for 5-7 minutes until browned.
Stir in the sliced mushrooms and cook for another 3-4 minutes until softened.
Add the cabbage and stir until it wilts, about 5-7 more minutes.
Pour in soy sauce, oyster sauce (if using), and sesame oil. Stir well to combine. Season with salt and pepper to taste.
Remove from heat and mix in chopped green onions.
Serve hot over cooked rice, garnishing with extra green onions if desired.