In a small bowl, whisk the brown sugar, soy sauce, sesame oil, red pepper flakes, and ginger together until well combined.
Heat the vegetable oil in a large cast iron skillet over medium high heat. Add the garlic and cook, stirring constantly, until fragrant, about 1 minute. Add the ground beef and cook for 3 to 5 minutes, breaking it apart as it browns. Drain off any excess fat.
Add the soy sauce mixture and green onions to the skillet and stir until everything is evenly combined. Let it simmer for about 2 minutes, just until heated through.
Serve right away, topped with extra green onion and a sprinkle of sesame seeds if you like.
