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 minced ginger to the skillet. Sauté for 2-3 minutes until the onion is translucent.

Stir in the shredded cabbage and cook for another 5-7 minutes, stirring occasionally, until the cabbage is wilted and tender.

In a small bowl, mix together the soy sauce, rice vinegar, brown sugar, and red pepper flakes. Pour this mixture over the beef and cabbage, stirring well to combine. Season with salt and pepper to taste.

Cook for an additional 2-3 minutes to heat through and allow the flavors to meld.

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