Slice the flank steak into very thin strips against the grain to ensure tenderness.
In a small bowl, whisk together the coconut aminos, water, arrowroot powder, and toasted sesame oil until smooth.
Heat the avocado oil in a large skillet or wok over medium-high heat until shimmering.
Add the beef strips in a single layer, seasoning with sea salt and black pepper, and sear for 2 minutes until browned.
Toss in the broccoli florets, minced ginger, and garlic, then stir-fry for 3-4 minutes until the broccoli is bright green and tender-crisp.
Pour the sauce mixture over the beef and vegetables, tossing constantly for 1 minute until the sauce thickens into a glossy glaze.
Remove from heat and garnish with thinly sliced green onions before serving.