Trim excess fat from flank steak, slice thinly against the grain, aim for ¼ inch thick strips.

In a medium bowl, whisk together soy sauce, water, apple cider vinegar, toasted sesame oil, brown sugar, cornstarch, ginger, garlic, and crushed red pepper flakes.

Heat 1 tablespoon olive oil in a large skillet or wok over high heat. Add broccoli, stir-fry for 4-5 minutes until bright green and tender-crisp, then remove and set aside.

In the same pan, add remaining olive oil, increase heat. Sear beef in a single layer for about 1 minute, then flip/stir and cook another minute until browned.

Return cooked broccoli to the skillet, pour sauce over mixture, gently toss to combine ensuring every piece is coated.

Cook over medium-high heat for 1-3 minutes until sauce thickens and becomes glossy.

Remove from heat, serve hot, garnished with sesame seeds.