Make the sauce. In a small bowl, whisk soy sauce, brown sugar, rice vinegar, sesame oil, cornstarch, and water or broth until smooth. Add a pinch of red pepper flakes if you like heat. Set aside.

Prep aromatics. Mince garlic, grate ginger, and slice green onions. Keep the white parts separate from the green tops for finishing.

Brown the beef. Heat oil in a large skillet over medium-high. Add ground beef and break it up. Cook until well-browned with some crispy bits, about 6–8 minutes. Spoon off excess fat if needed, but leave a bit for flavor.

Add aromatics. Stir in garlic, ginger, and the white parts of the green onions. Cook 1–2 minutes until fragrant, stirring so nothing scorches.

Optional veggies. If using quick-cooking vegetables (like shredded carrots or snap peas), add them now and cook 2–3 minutes until crisp-tender. For broccoli, steam or blanch first, then toss in.

Sauce it up. Give the sauce a quick stir (cornstarch settles), then pour it into the pan. Stir well and let it bubble for 2–3 minutes until glossy and slightly thickened, coating the beef.

Finish and taste. Turn off the heat. Stir in most of the green onion tops, reserving some for garnish. Taste and adjust: more soy for salt, more vinegar for brightness, or a pinch more sugar to balance.

Serve. Spoon over hot rice or noodles. Garnish with sesame seeds and the remaining green onions. Add extra chili flakes if you like it spicy.