In a large skillet or wok, heat the sesame oil over medium-high heat. Add the ground beef and cook until browned, breaking it apart with a spatula, about 5-7 minutes. Drain excess fat if necessary.

Add the sliced onion, minced garlic, and ginger to the skillet. Stir-fry for about 2-3 minutes until the onion becomes translucent.

Stir in the shredded cabbage and cook for another 5 minutes, stirring frequently, until the cabbage is tender but still crisp.

In a small bowl, mix together the soy sauce, rice vinegar, sugar, black pepper, and red pepper flakes (if using). Pour this sauce over the beef and cabbage mixture. Stir well to combine and cook for an additional 2 minutes.

Remove from heat and garnish with sliced green onions before serving.