Place the flank steak on a cutting board and using a meat mallet or the back of a skillet, tenderize the meat, pounding the steak until it is [1/2] inch thick. Remove and discard the parchment paper, then slice the steak against the grain into [1/4]-inch-thin pieces. Add the steak to a large bowl and sprinkle with the salt, pepper, and arrowroot starch. Toss to coat.
In a large nonstick skillet, heat the avocado oil over medium-high heat. Working in batches so you don’t crowd the pan, add the steak in a single layer. Sear the steak until it forms a deep brown crust, 3 to 4 minutes per side. Transfer the cooked steak to a plate and set aside. Repeat until all the steak is cooked and all of the meat is set aside.
Add the garlic, ginger, sesame oil, and red pepper flakes to the skillet and cook, stirring, for 1 minute. Pour in the beef broth and bring to a simmer. Let simmer for about 2 minutes and use the back of a spoon to scrape up any brown bits from the pan.
Return the steak to the skillet. Stir in the green onions, coconut aminos, rice vinegar, and fish sauce and bring the sauce to a simmer. Cook, stirring often, until the sauce has thickened, 3 to 5 minutes.
Spoon the stir fry over the Cauliflower Rice and sprinkle with the sesame seeds. Serve with the Seared Baby Bok Choy.