Slice the flank steak against the grain into thin, bite-sized strips to ensure maximum tenderness.
In a small bowl, whisk together the tamari, water, minced ginger, minced garlic, red pepper flakes, and arrowroot powder until smooth.
Heat the toasted sesame oil in a large skillet or wok over medium-high heat until it begins to shimmer.
Add the beef strips in a single layer and sear for 2 minutes without moving them to develop a deep brown crust, then flip and cook for 1 more minute.
Remove the beef from the pan and set aside on a plate.
Add the broccoli florets to the same skillet with a splash of water, cover immediately, and steam for 3 minutes until vibrant green and tender-crisp.
Return the beef and any accumulated juices to the pan and pour the sauce mixture over the top.
Toss everything constantly for 1 minute until the sauce thickens into a glossy glaze that coats the beef and broccoli.
Garnish with sesame seeds and serve immediately while hot.