Whisk the sauce: In a small bowl, whisk soy sauce, oyster sauce, rice vinegar, brown sugar, broth, cornstarch, and sesame oil until smooth. Set aside.
Prep the broccoli: If using fresh florets, cut them into small, even pieces for quick cooking.
If using frozen, thaw and pat dry to avoid excess water.
Brown the beef: Heat oil in a large skillet over medium-high heat. Add ground beef and cook, breaking it up, until well browned and most moisture has cooked off, about 5–6 minutes. Season lightly with salt and pepper only if needed.
Add aromatics: Stir in garlic, ginger, red pepper flakes (if using), and the white parts of the green onions.
Cook 30–60 seconds until fragrant.
Cook the broccoli: Add broccoli to the skillet. Toss with the beef and aromatics. If your pan seems dry, splash in 2 tablespoons water, cover for 1–2 minutes to steam, then uncover.
Sauce and simmer: Give the sauce a quick stir, then pour it into the skillet.
Toss to coat. Simmer 1–2 minutes, stirring, until glossy and thickened. The broccoli should be bright green and crisp-tender.
Finish and garnish: Remove from heat.
Stir in the green parts of the green onions. Taste and adjust—add a splash more soy for salt, a pinch of sugar for balance, or a squeeze of lime for brightness. Garnish with sesame seeds.
Serve: Spoon over rice or noodles, or keep it low-carb with cauliflower rice.
Enjoy immediately.