Best Marinated Flank Steak (or flat iron steak)

This recipe is so simple that it's hard to believe how good it is, but it's my new favorite way to prepare beef before grilling it.  I found the recipe here, but I modified it to use fish sauce instead of oyster sauce, to which I may be allergic.  You might be worried that the beef will end up tasting fishy, but it really doesn't.  Somehow the fish sauce, combined with the other ingredients, just gives it a great depth of "umami" flavor.  And in addition to loving the taste, I love the word "umami," don't you?

Ingredients:

• 1 1/2 lb. flank steak or flat iron steak
• 1/4 cup fish sauce.  Buy one that is labeled gluten-free, like the Thai Kitchen brand.
• 2 Tbsp. balsamic vinegar
• 1 tsp. minced garlic
• 1 tsp. grainy mustard or Dijon mustard
• black pepper -- to sprinkle on before grillin

Instructions:

Cut any excess fat or silver skin off of the steak.  Whisk together the fish sauce, balsamic vinegar, garlic, and mustard in a small bowl.  Place the steak in a gallon-size Zip-loc bag and pour the marinate into the bag.  Seal the bag and massage the marinade into the steak.  Place the steak in the refrigerator and marinate for at least 4 hours, but up to overnight.  Remove the steak from the bag and sprinkle it with black pepper.  Grill until medium done.  Let the steak rest 10 minutes before slicing it thinly against the grain.