In a small bowl, whisk together soy sauce, brown sugar, sesame oil, minced garlic, grated ginger, and red pepper flakes (if using). Set aside.
Heat a large skillet or wok over medium-high heat. Add the ground beef and cook, breaking it into crumbles, until no longer pink and the internal temperature reaches 160°F (71°C), about 5-6 minutes. Drain any excess fat.
Pour the sauce over the beef and stir to coat. Cook for 2-3 minutes, allowing the sauce to bubble and slightly caramelize, coating the meat beautifully.
Remove from heat, stir in half of the sliced green onions. Serve immediately over steamed rice, garnished with remaining green onions and a sprinkle of sesame seeds.