In a medium bowl, combine the thinly sliced flank steak with 1 tbsp soy sauce, 1 tbsp cornstarch, 1 tsp sesame oil, and 2 tbsp water. Mix well and let marinate for at least 15 minutes at room temperature, or up to 30 minutes in the refrigerator.

While the beef marinates, prepare the broccoli. You can steam it for 3-5 minutes until tender-crisp, or blanch it in boiling water for 1-2 minutes then immediately transfer to an ice bath to stop cooking. Drain well and set aside.

In a small bowl, whisk together the beef broth (or water), 1/4 cup soy sauce, oyster sauce (if using), brown sugar, 1 tsp sesame oil, and 1 tbsp cornstarch until smooth. Set the sauce aside.

Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add half of the marinated beef in a single layer and cook for 2-3 minutes per side until browned. Remove cooked beef from the pan and set aside. Add the remaining 1 tablespoon of vegetable oil and cook the rest of the beef, then remove and set aside.

In the same skillet or wok, add the minced garlic and grated ginger. Sauté for 30 seconds until fragrant, being careful not to burn the garlic.

Return all the cooked beef to the skillet. Add the prepared broccoli florets to the pan.

Give the prepared sauce a quick whisk again, then pour it over the beef and broccoli. Cook, stirring constantly, for 1-2 minutes until the sauce thickens and coats all the ingredients.

Remove from heat and serve immediately over steamed rice.