In a bowl, whisk together soy sauce, brown sugar, minced garlic, grated ginger, rice vinegar, and sesame oil until smooth, about 2 minutes.

In a separate bowl, mix cornstarch with water until completely smooth.

Heat a large skillet over medium-high heat for about 2 minutes. Add the ground beef and cook for 5–7 minutes, breaking it apart until it's browned.

After draining excess fat, add diced onion and chopped red bell pepper. Stir-fry for 3–4 minutes until softened.

Stir in broccoli florets and cook for 2–3 minutes until bright green and tender-crisp.

Pour the prepared teriyaki sauce into the skillet and stir well. Cook for 2 minutes to meld flavors.

Add the cornstarch slurry and stir continuously for 1–2 minutes until the sauce thickens.

Remove from heat and sprinkle with sesame seeds and chopped green onions. Serve hot over cooked rice.