Heat the vegetable oil in a large skillet or wok over medium-high heat.
Add the ground beef to the skillet and cook, breaking it apart with a spatula, until browned and cooked through, about 5-7 minutes.
Remove the cooked ground beef from the skillet and set aside. Drain excess fat if necessary, leaving about 1 tablespoon in the skillet.
In the same skillet, add the minced garlic and ginger and sauté for 30 seconds until fragrant.
Add the sliced onion and cook for 2-3 minutes until softened.
Add the sliced cabbage to the skillet and stir-fry for 5-7 minutes until the cabbage is tender but still slightly crisp.
Return the cooked ground beef to the skillet with the vegetables.
Add the soy sauce, oyster sauce, black pepper, and red pepper flakes. Stir well to combine all ingredients and cook for an additional 2 minutes to heat through.
Drizzle the sesame oil over the stir fry and toss to coat evenly.
Remove from heat and garnish with sliced green onions before serving.