Combine soy sauce, rice vinegar, brown sugar, white pepper, and chili flakes in a small bowl. Stir until sugar dissolves. Set aside.
Heat oil in a large skillet over medium-high heat. Cook diced onion for 4 to 5 minutes until golden and softened.
Push onion to the edges. Cook garlic and ginger in the center for 45 to 60 seconds until fragrant.
Add ground beef. Break apart and cook 7 to 8 minutes until browned and cooked through to 75°C (165°F) internal temperature.
Drain excess fat, leaving about 1 teaspoon in the pan.
Add coleslaw mix and toss with tongs. Cook 4 to 5 minutes over medium-high heat until cabbage is wilted but still has some bite.
Pour sauce over the pan. Toss to coat and cook 1 to 2 minutes until absorbed.
Remove from heat. Drizzle sesame oil over the pan and toss once.
Serve in bowls topped with sliced green onions and sesame seeds.