In a mixing bowl, whisk together the soy sauce, brown sugar, minced garlic, grated ginger, rice vinegar, and sesame oil.

In a separate bowl, mix the cornstarch with water until you achieve a smooth slurry.

Heat a large skillet over medium-high heat and add the ground beef, breaking it apart as it cooks until nicely browned, about 5-7 minutes.

Add the diced onion and red bell pepper to the skillet and sauté for 3-4 minutes until the onion becomes translucent.

Incorporate the broccoli florets and cook for another 2-3 minutes.

Pour the prepared sauce mixture over the beef and vegetables and stir everything together thoroughly.

Quickly add the cornstarch slurry to the skillet while stirring continuously, cooking until the sauce thickens, about 1-2 minutes.

Garnish with sesame seeds and chopped green onions, and serve over hot cooked rice.