Prep the beef. Pat beef slices dry with paper towels. Toss with cornstarch until evenly coated. This helps tenderize the beef and creates a velvety texture.

Make the sauce. In a small bowl, whisk together soy sauce, oyster sauce, brown sugar, rice vinegar, beef broth, and sesame oil. Set aside.

Cook the beef. Heat 2 tablespoons of vegetable oil in a large skillet or wok over high heat. Add the beef in a single layer (work in batches if needed) and cook for 2-3 minutes without stirring, allowing it to sear. Flip and cook for another 1-2 minutes until browned but not fully cooked through. Remove beef from the skillet and set aside.

Cook the broccoli. Add the remaining 1 tablespoon of oil to the same skillet. Add broccoli florets and stir-fry for 3-4 minutes until bright green and tender-crisp. Add a splash of water if needed to help steam the broccoli.

Add aromatics. Push the broccoli to the side of the skillet. Add garlic, ginger, and red pepper flakes to the center and cook for 30 seconds until fragrant.

Combine everything. Return the beef to the skillet along with any accumulated juices. Pour the sauce over everything and toss to combine. Cook for 2-3 minutes, stirring frequently, until the sauce thickens and coats the beef and broccoli.

Serve. Remove from heat and serve immediately over cooked rice. Garnish with sesame seeds and sliced green onions.