Preheat oven to 450°F and line a sheet pan with parchment paper for best results.
In a medium bowl, whisk together soy sauce, honey, rice vinegar, sesame oil, minced garlic, grated ginger, and cornstarch until smooth to make the teriyaki sauce.
Toss sliced flank steak with half of the teriyaki sauce and let it sit while you prep the vegetables.
In another bowl, toss broccoli florets and bell pepper strips with vegetable oil, salt, and pepper.
Spread vegetables on the sheet pan and roast for 8 minutes until they start to char slightly.
Remove pan from oven, push vegetables to the sides, and arrange marinated beef in a single layer in the center of the pan.
Return to oven and roast for 6-8 minutes until beef is cooked through and edges are caramelized, then drizzle with remaining teriyaki sauce and garnish with sesame seeds and sliced green onions before serving.