In a bowl, whisk together the beef broth, soy sauce, brown sugar, garlic, ginger, red pepper flakes, and cornstarch. Set aside.
Heat olive oil in a large skillet or wok over medium-high heat. Add the broccoli and season with salt and pepper. Cook for 3–5 minutes until tender and lightly browned. Remove from the pan and set aside.
Add the ground beef and diced onion to the skillet. Cook until the beef is fully browned, breaking it apart as it cooks. Season with additional salt and pepper if desired.
Return the broccoli to the skillet. Pour in the sauce mixture and stir well. Cook for a few minutes until the sauce thickens and coats everything evenly.
Garnish with green onion and serve warm over rice if desired.