Season the beef to taste with sea salt and freshly ground black pepper.
Warm the coconut oil in a large skillet placed over a high heat.
Add the ginger, garlic, and chiles (if using).
Stir-fry until fragrant, about 1 minute.
Add the beef and cook for 2 to 3 minutes.
Transfer the beef to a bowl.
Add the onion to the hot skillet and cook for 2 minutes.
Add the bok choy and cook until soft (3 to 4 minutes).
Return the beef to the skillet, add the fish sauce to the pan, stir to combine, and serve warm.