Slice the flat iron steak very thinly against the grain.
Place the sliced beef in a resealable bag or covered bowl. Add the coconut aminos, grated fresh ginger, and chopped garlic. Mix until the beef is evenly coated.
Refrigerate and allow the beef to marinate for 1 hour.
When ready to cook, remove the beef from the marinade and reserve the marinade to use as the stir-fry sauce.
Bring a pot of water to a boil. Add the broccoli and blanch for 2 minutes. Drain thoroughly.
Heat the coconut oil in a large wok or cast-iron skillet over medium-high heat.
Add the drained beef and stir-fry for 1–3 minutes, or until nicely browned. Work in batches if necessary to avoid overcrowding the pan. Remove the beef and set aside.
Add the broccoli to the hot pan and stir-fry for about 3 minutes, until crisp-tender.
Pour the reserved marinade into the skillet and cook with the broccoli for about 2 minutes, making sure the sauce comes to a full simmer.
Return the cooked beef to the skillet and toss everything together until hot and evenly coated.
Stir in the fish sauce and toasted sesame oil. Serve your Keto Beef and Broccoli immediately while the beef is tender and the broccoli is still crisp.
Notes
Flat iron steak works particularly well because it cooks quickly and stays tender when thinly sliced. Flank steak or sirloin can also be used.