Heat a large skillet over medium heat and add the ground beef. Cook until browned, about 3 to 5 minutes, making sure to break up the beef as it cooks.
Stir in the garlic and cook until fragrant, about 1 minute.
Whisk together the brown sugar, soy sauce, sesame oil, ginger, and red pepper flakes in a small bowl.
Pour the sauce over the cooked beef and let it simmer for 2 to 3 minutes until the flavors combine.
Serve the beef over warm rice and garnish with sliced green onions.