In a small bowl, whisk together the soy sauce, beef broth, brown sugar, oyster sauce (if using), rice vinegar, sesame oil, cornstarch, ginger, and garlic. Set aside.

Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add the broccoli and cook for 3–4 minutes, stirring occasionally, until it turns bright green and just starts to soften. Transfer to a plate.

Add the remaining oil to the skillet, then add the ground beef. Cook, breaking it apart as it browns.

Once the beef is mostly cooked, add the diced onion and cook for another 3–4 minutes until softened.

Return the broccoli to the pan and stir everything together.

Pour in the sauce and bring to a simmer. Cook for 2–3 minutes, stirring frequently, until the sauce thickens and coats everything evenly.

Remove from heat and stir in the green onions.