Thinly slice flank steak against the grain. In a medium bowl, combine the beef with soy sauce, cornstarch, vegetable oil, sesame oil, ground ginger, garlic powder, and black pepper. Mix well, cover with plastic wrap, and refrigerate for at least 30 minutes.

Cut broccoli into florets. In a skillet, heat vegetable oil over medium heat. Sauté minced garlic for 30 seconds, then add broccoli and stir-fry for 2 minutes. Add splash of water and cover for 3-5 minutes until tender-crisp. Set aside.

In a bowl, whisk together beef broth, soy sauce, brown sugar, oyster sauce, cornstarch, sesame oil, ground ginger, and red pepper flakes until smooth. Set aside.

Heat skillet over high heat and add vegetable oil. Stir-fry marinated beef in batches for 2-3 minutes until browned. Remove beef from skillet and set aside.

Pour sauce into the skillet and bring to a simmer over medium-high heat. Cook for 1-2 minutes until sauce thickens.

Return broccoli to the skillet, mix well with sauce and beef. Cook for 1-2 minutes until heated through.

Serve over cooked rice and garnish with sesame seeds and green onions.