In a large skillet or wok, heat the vegetable oil over medium-high heat. Add the diced onion and sauté for about 2-3 minutes until translucent.
Add the minced garlic and grated ginger to the skillet, cooking for an additional 1 minute until fragrant.
Increase the heat to high and add the ground beef to the skillet. Cook, breaking it apart with a spatula, until browned and cooked through, about 5-7 minutes.
Stir in the shredded cabbage, soy sauce, oyster sauce, sesame oil, and red pepper flakes (if using). Cook for another 5-7 minutes, stirring frequently, until the cabbage is tender but still crisp.
Season with salt and pepper to taste. Remove from heat and garnish with sliced green onions before serving.