Slice the flank steak against the grain into thin strips and season evenly with sea salt and black pepper.
In a small glass bowl, whisk together the tamari, arrowroot powder, minced garlic, and grated ginger until smooth.
Heat the sesame oil in a large stainless steel skillet or wok over medium-high heat until it begins to shimmer.
Add the beef strips to the hot pan in a single layer, searing for 2 minutes until a golden crust forms, then remove the beef and set aside.
Add the broccoli florets and sliced red bell pepper to the same pan, stirring frequently for 3-4 minutes until they are tender-crisp and bright.
Return the beef and any accumulated juices to the pan and pour the tamari sauce over the mixture.
Toss everything constantly for 1 minute until the sauce thickens into a glossy glaze that coats the beef and vegetables.
Garnish with sesame seeds and serve immediately while hot.