Slice the flank steak against the grain into thin, bite-sized strips.
In a small bowl, whisk together the tamari, beef broth, grated ginger, and arrowroot powder until smooth.
Heat the sesame oil in a large skillet or wok over medium-high heat until shimmering.
Add the beef strips in a single layer and sear until browned, about 2 minutes per side, then remove and set aside.
Add the broccoli florets and minced garlic to the same skillet with a splash of water, covering for 2 minutes to steam until bright green.
Return the beef to the pan, pour in the sauce mixture, and sprinkle with red pepper flakes, sea salt, and black pepper.
Toss everything together for 1-2 minutes until the sauce has thickened and coats the beef and broccoli beautifully.