In a medium bowl, toss the thinly sliced beef with 1 tablespoon of cornstarch until evenly coated. Set aside.

In a separate bowl or measuring cup, whisk together the remaining 1 tablespoon cornstarch, soy sauce, oyster sauce, brown sugar, rice vinegar, and water or broth. This is your stir-fry sauce.

Heat 1 tablespoon of vegetable oil in a large wok or skillet over high heat. Add broccoli florets and stir-fry for 3-4 minutes until bright green and crisp-tender. Remove broccoli to a plate.

Add the remaining 2 tablespoons of oil to the hot wok. Add the marinated beef in a single layer (cook in batches if necessary) and sear for 1-2 minutes per side until browned. Remove beef to the plate with the broccoli.

Reduce heat to medium. Add minced garlic and grated ginger to the wok and stir for 30 seconds until fragrant.

Give the prepared sauce a quick re-stir and pour it into the wok. Cook, stirring constantly, until the sauce thickens and becomes glossy, about 1-2 minutes.

Return the cooked beef and broccoli to the wok. Toss everything together until thoroughly coated in the rich sauce. Drizzle with sesame oil and remove from heat.

Garnish with sesame seeds and green onions before serving immediately.