Prepare rice: Cook rice according to package directions. White rice takes 15 minutes, brown rice needs 40 to 45 minutes. Keep warm until ready to serve.
Make sauce: In a small bowl, whisk together brown sugar, soy sauce, sesame oil, grated ginger, red pepper flakes, and black pepper until combined. Set aside.
Cook beef: Heat a large skillet over medium-high heat. Add ground beef and cook for 5 to 7 minutes, breaking into small crumbles with a wooden spoon. Continue cooking until no pink remains and beef is browned.
Add garlic: Stir in minced garlic and cook for 30 seconds until fragrant. Don't let garlic burn.
Add sauce: Pour prepared sauce over beef. Add half the sliced green onions. Stir well and simmer for 2 to 3 minutes, stirring occasionally, until sauce thickens and becomes glossy.
Assemble bowls: Divide cooked rice among four bowls. Top each with Korean beef mixture. Garnish with remaining green onions, sesame seeds, and optional vegetables.