Toss the sliced beef with 1 tablespoon cornstarch until evenly coated and let rest for 15 minutes.
Whisk together soy sauce, brown sugar, hoisin sauce, ginger, and remaining 1 tablespoon cornstarch in a small bowl.
Heat vegetable oil in a wok or large skillet over high heat until shimmering.
Add the beef in a single layer and sear for 1 to 2 minutes per side until browned but still tender.
Pour the sauce into the wok and toss with the beef for 1 to 2 minutes until the sauce thickens and coats the meat.
Add the green onions and toss for 30 seconds until just softened, then serve immediately over rice.