Brown the ground beef: Heat a large skillet over medium-high heat. Add the ground beef and break it apart with a wooden spoon or spatula. Cook for 7-8 minutes until the meat is fully browned with no pink remaining. If using higher-fat beef, drain off most of the excess fat, leaving about 1 tablespoon for flavor.
Prepare the teriyaki sauce: While the beef cooks, add all sauce ingredients (brown sugar, grated ginger, minced garlic, mirin, soy sauce, rice wine vinegar, beef broth, and cornstarch) to a medium bowl. Whisk thoroughly until the cornstarch is completely dissolved with no lumps remaining.
Add the vegetables: To the skillet with the browned beef, add the chopped bell pepper, matchstick carrots, and scallions. Stir-fry for 3 minutes until the vegetables begin to soften but still retain a crisp texture. Stir frequently to prevent burning.
Pour in the sauce: Pour the prepared teriyaki sauce over the beef and vegetables. Stir everything together until well coated. Let the mixture cook for 3-4 minutes, stirring occasionally, until the sauce thickens and becomes glossy, clinging to the beef and vegetables.
Taste and adjust: Taste the teriyaki ground beef and adjust seasonings as needed. Add more soy sauce if it's too sweet, a pinch of brown sugar if too salty, or red pepper flakes for heat. Season with salt and pepper to taste if desired.
Serve: Serve the teriyaki ground beef over freshly cooked rice or noodles. Garnish with toasted sesame seeds and extra sliced scallions if desired. For a low-carb option, serve over cauliflower rice or in lettuce wraps.