In a small bowl, whisk together brown sugar, soy sauce, sesame oil, red pepper flakes, and ginger.

Heat vegetable oil in a large cast iron skillet over medium-high heat. Add garlic and cook, stirring constantly, until fragrant, about 1 minute.

Add ground beef and cook until browned, about 3-5 minutes, making sure to crumble the beef as it cooks; drain excess fat.

Stir in soy sauce mixture and green onions until well combined, allowing to simmer until heated through, about 2 minutes.

Serve immediately, garnished with green onion and sesame seeds, if desired.