Prep the steak: Pat the beef dry with paper towels.
Slice thinly against the grain. Season lightly with black pepper. Set aside while you prep the sauce.
Make the sauce: In a bowl, whisk together coconut aminos, rice vinegar, sesame oil, fish sauce, sweetener, and beef broth.
Sprinkle in xanthan gum while whisking to avoid clumps. Set aside. The sauce will thicken more as it cooks.
Cook the broccoli: Heat 1 tablespoon oil in a large skillet over medium-high heat.
Add broccoli and a pinch of salt. Stir-fry 3–4 minutes until crisp-tender with a bit of char. Transfer to a plate.
Sear the beef: Add the remaining 1 tablespoon oil.
Increase heat to high. Spread the beef in a single layer and let it sear 1–2 minutes without moving. Stir and cook another 1–2 minutes until mostly browned but not fully cooked through.
Work in batches if needed to avoid steaming.
Aromatics in: Reduce heat to medium. Add minced garlic, grated ginger, red pepper flakes, and the white parts of the green onions. Sauté 30–45 seconds until fragrant.
Sauce it up: Whisk the sauce again and pour it into the skillet.
Toss with the beef and bring to a gentle simmer for 1–2 minutes until glossy and slightly thickened.
Finish with broccoli: Return the broccoli to the pan. Toss to coat and cook 1–2 more minutes, just until everything is heated through and the beef is tender.
Taste and adjust: Add more coconut aminos for salt, a splash of vinegar for brightness, or more red pepper flakes for heat. Top with the green onion tops and sesame seeds if using.
Serve: Enjoy as is, or spoon over cauliflower rice or shirataki rice.
A squeeze of lime brightens it up nicely.