Pound the flanks steak until it's .5" thick. Slice into .25" strips. Add to a bowl with salt, pepper and arrowroot. Toss to coat.
In a large nonstick skillet, heat oil (medium-high). Sear the steak until it has a deep brown crust (3-4 min/side). Do this in batches with a single layer of meat. Transfer steak to a plate as it's browned.
Add ginger, garlic, sesame oil anr pepper flakes to the skillet - stir for 1 minute. Pour in broth and bring to a simmer - let it simmer for ~2 minutes. Scrape up any brown bits from the pan.
Return the steak to the skillet. Stir in green onions, coconut aminos, rice vinegar and fish sauce - bring back to a simmer. Cook until thickened, stirring often - maybe 3-5 minutes.