(Optional) Velvet the beef: Toss the sliced beef with the baking soda and let sit 15–30 minutes, then rinse and pat dry. This keeps the beef extra tender.
Make the sauce: In a bowl, whisk together the soy sauce, brown sugar, oyster sauce, sesame oil, garlic, and ginger. In a separate cup, stir the cornstarch into the water or beef broth to make a slurry.
Marinate: Place the beef in a bowl and pour over half the sauce. Let marinate at least 15 minutes.
Preheat: Heat the Blackstone griddle to medium-high (350–375°F) and lightly oil the surface.
Sear the beef: Spread the beef in a single layer and sear 2–3 minutes per side, until browned. Don't crowd the griddle — work in batches if needed. Remove and set aside.
Cook the broccoli: Add a little more oil, then the broccoli. Stir-fry 3–4 minutes until bright green and tender-crisp. (For softer broccoli, splash with water and cover with a dome 1–2 minutes.)
Combine: Return the beef to the griddle. Pour over the remaining sauce and the cornstarch slurry. Toss and cook 1–2 minutes until the sauce thickens and coats everything in a glossy glaze.
Serve: Garnish with green onions and serve hot over rice or noodles.