In a small bowl, whisk together brown sugar, soy sauce, sesame oil, crushed red pepper flakes, and ground ginger.
Heat vegetable oil in a large cast iron skillet over medium-high heat.
Add minced garlic and cook, stirring constantly, until fragrant, about 1 minute.
Add ground beef to the skillet and cook until browned, about 3 to 5 minutes, crumbling the beef as it cooks. Drain excess fat.
Stir in the soy sauce mixture and green onions, combining well, then simmer until heated through, about 2 minutes.
Serve immediately, garnished with green onions and sesame seeds if desired.