Prep the broccoli: Cut broccoli into bite-size florets.
If using fresh, rinse and shake dry. If using frozen, thaw and pat dry to avoid watering down the sauce.
Make the sauce base: In a small bowl, whisk soy sauce, oyster sauce, brown sugar, rice vinegar, red pepper flakes, and beef broth or water. Set aside.
Brown the beef: Heat 1 tablespoon oil in a large skillet over medium-high heat.
Add ground beef. Season lightly with salt and pepper, and cook, breaking it up, until browned and no longer pink, about 5–7 minutes. Drain excess fat if needed.
Sauté aromatics: Push beef to the edges of the pan.
Add remaining oil to the center. Add onion (if using), garlic, and ginger. Sauté 1–2 minutes until fragrant, then mix with the beef.
Cook the broccoli: Add broccoli to the skillet.
Pour in a splash of water (2–3 tablespoons), cover, and steam for 2–3 minutes until bright green and just tender.
Add the sauce: Pour the sauce base over the beef and broccoli. Stir to coat. Bring to a gentle simmer.
Thicken: Stir in the cornstarch slurry.
Cook 1–2 minutes, stirring, until the sauce turns glossy and clings to the meat and broccoli.
Finish and taste: Turn off the heat. Stir in sesame oil. Taste and adjust seasoning—add more soy for salt, sugar for sweetness, or vinegar for brightness.
Serve: Spoon over warm rice or toss with noodles.
Garnish with green onions and sesame seeds if you like.