Slice the flank steak against the grain into thin strips about 1/4-inch thick. In a small bowl, whisk together all sauce ingredients except the cornstarch slurry. Mix the cornstarch with 2 tablespoons water in a separate bowl. Have all vegetables and aromatics ready to go.

Heat 1 tablespoon of vegetable oil in a wok or large skillet over high heat until shimmering and nearly smoking. Add the beef in a single layer and let it sear undisturbed for 1 minute. Stir and cook until just browned but still slightly pink in spots, about 1-2 more minutes. Transfer to a plate.

Add the remaining 1 tablespoon oil to the hot pan. Toss in the broccoli and bell pepper. Stir-fry for 2-3 minutes until the broccoli turns bright green and is crisp-tender. Add the garlic and ginger and stir for 30 seconds until fragrant.

Return the beef (and any accumulated juices) to the pan. Pour in the sauce mixture and the cornstarch slurry. Stir everything together and cook for 1-2 minutes until the sauce thickens and coats the beef and veggies evenly. Ensure the beef reaches an internal temperature of 145°F (63°C) for food safety; use an instant-read thermometer.

Remove from heat, drizzle with a little extra toasted sesame oil if desired, and transfer to a serving platter. Garnish with green onions and sesame seeds. Pile over rice or noodles and eat while sizzling hot.