Prepare all ingredients by shredding cabbage, slicing onion, and mincing garlic and ginger.
Heat sesame oil in a large skillet or wok over medium-high heat. Add ground beef and cook for 5-7 minutes until browned.
Add sliced onion, minced garlic, and ginger to the skillet. Stir-fry for 3-4 minutes until onions are translucent.
Toss in shredded cabbage and cook for another 5 minutes, stirring occasionally.
Mix soy sauce, brown sugar, and red pepper flakes in a small bowl. Pour over beef and cabbage, stir well, and cook for 2-3 minutes.
Season with salt and pepper to taste, garnish with sliced green onions, and serve.