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