Preheat oven to 450°F (230°C). Line a large rimmed sheet pan with foil and spray with cooking spray.
In a small bowl, whisk together all sauce ingredients (soy sauce, oyster sauce, brown sugar, cornstarch, ginger, garlic, sesame oil, and red pepper flakes). Set aside.
Toss broccoli florets with 1 tablespoon sesame oil and a pinch of salt. Spread in a single layer on the prepared sheet pan.
Roast broccoli for 10-12 minutes until edges are crispy and beginning to char.
While broccoli roasts, toss sliced flank steak with 1 tablespoon sesame oil and season lightly with pepper. Pour half the sauce over the beef and toss to coat.
Push broccoli to the sides of the pan. Add the beef in a single layer in the center of the pan.
Drizzle the remaining sauce over the broccoli and beef.
Return to oven and roast for 6-8 minutes until beef is cooked through and caramelized at the edges.
Sprinkle with sesame seeds and sliced green onions. Serve immediately over white rice.