Place the steak(s) In a shallow baking pan or dish that will fit the steak OR in a large ziplock bag.  Mix the soy sauce, brown sugar, garlic and water together in a bowl and add to the flank steak. Cover the dish, refrigerate and marinate for at least 2 hours or overnight, turning once if needed (you will need to do this if it's in a pan).

Remove the flank steak from the marinade and place on hot BBQ for 5 minutes on each side or until cooked to your preference. 

Remove the steak from the barbecue, cover with aluminum foil and allow to rest for 5 minutes before slicing, using a sharp chef's knife, on a diagonal (around a 22 degree angle) into thin pieces. Serve immediately with the feta salsa.

Combine all of the ingredients in a medium bowl and stir until well combined. Refrigerate until ready to use. You can make this ahead and store it in the fridge for up to 3 days.